Remove unnecessary `explicit` qualifiers on constructors.

Change-Id: Id12e392ad50f66a6e2251a68662b7959315dc567
diff --git a/runtime/gc/collector/concurrent_copying.cc b/runtime/gc/collector/concurrent_copying.cc
index 8e329d6..220c06e 100644
--- a/runtime/gc/collector/concurrent_copying.cc
+++ b/runtime/gc/collector/concurrent_copying.cc
@@ -181,7 +181,7 @@
 // Used to switch the thread roots of a thread from from-space refs to to-space refs.
 class ThreadFlipVisitor : public Closure {
  public:
-  explicit ThreadFlipVisitor(ConcurrentCopying* concurrent_copying, bool use_tlab)
+  ThreadFlipVisitor(ConcurrentCopying* concurrent_copying, bool use_tlab)
       : concurrent_copying_(concurrent_copying), use_tlab_(use_tlab) {
   }
 
@@ -817,8 +817,8 @@
 
 class RevokeThreadLocalMarkStackCheckpoint : public Closure {
  public:
-  explicit RevokeThreadLocalMarkStackCheckpoint(ConcurrentCopying* concurrent_copying,
-                                                bool disable_weak_ref_access)
+  RevokeThreadLocalMarkStackCheckpoint(ConcurrentCopying* concurrent_copying,
+                                       bool disable_weak_ref_access)
       : concurrent_copying_(concurrent_copying),
         disable_weak_ref_access_(disable_weak_ref_access) {
   }
diff --git a/runtime/gc/collector/mark_sweep.cc b/runtime/gc/collector/mark_sweep.cc
index e2bcca2..3b6d2aa 100644
--- a/runtime/gc/collector/mark_sweep.cc
+++ b/runtime/gc/collector/mark_sweep.cc
@@ -648,8 +648,8 @@
  protected:
   class MarkObjectParallelVisitor {
    public:
-    ALWAYS_INLINE explicit MarkObjectParallelVisitor(MarkStackTask<kUseFinger>* chunk_task,
-                                                     MarkSweep* mark_sweep)
+    ALWAYS_INLINE MarkObjectParallelVisitor(MarkStackTask<kUseFinger>* chunk_task,
+                                            MarkSweep* mark_sweep)
         : chunk_task_(chunk_task), mark_sweep_(mark_sweep) {}
 
     void operator()(mirror::Object* obj, MemberOffset offset, bool /* static */) const
@@ -1058,8 +1058,8 @@
 
 class CheckpointMarkThreadRoots : public Closure, public RootVisitor {
  public:
-  explicit CheckpointMarkThreadRoots(MarkSweep* mark_sweep,
-                                     bool revoke_ros_alloc_thread_local_buffers_at_checkpoint)
+  CheckpointMarkThreadRoots(MarkSweep* mark_sweep,
+                            bool revoke_ros_alloc_thread_local_buffers_at_checkpoint)
       : mark_sweep_(mark_sweep),
         revoke_ros_alloc_thread_local_buffers_at_checkpoint_(
             revoke_ros_alloc_thread_local_buffers_at_checkpoint) {
diff --git a/runtime/gc/collector/mark_sweep.h b/runtime/gc/collector/mark_sweep.h
index 606be63..8bd1dc7 100644
--- a/runtime/gc/collector/mark_sweep.h
+++ b/runtime/gc/collector/mark_sweep.h
@@ -54,7 +54,7 @@
 
 class MarkSweep : public GarbageCollector {
  public:
-  explicit MarkSweep(Heap* heap, bool is_concurrent, const std::string& name_prefix = "");
+  MarkSweep(Heap* heap, bool is_concurrent, const std::string& name_prefix = "");
 
   ~MarkSweep() {}
 
diff --git a/runtime/gc/collector/partial_mark_sweep.h b/runtime/gc/collector/partial_mark_sweep.h
index 7b69bce..e9b4f6f 100644
--- a/runtime/gc/collector/partial_mark_sweep.h
+++ b/runtime/gc/collector/partial_mark_sweep.h
@@ -30,7 +30,7 @@
     return kGcTypePartial;
   }
 
-  explicit PartialMarkSweep(Heap* heap, bool is_concurrent, const std::string& name_prefix = "");
+  PartialMarkSweep(Heap* heap, bool is_concurrent, const std::string& name_prefix = "");
   ~PartialMarkSweep() {}
 
  protected:
diff --git a/runtime/gc/collector/sticky_mark_sweep.h b/runtime/gc/collector/sticky_mark_sweep.h
index e8e70de..e8f0672 100644
--- a/runtime/gc/collector/sticky_mark_sweep.h
+++ b/runtime/gc/collector/sticky_mark_sweep.h
@@ -30,7 +30,7 @@
     return kGcTypeSticky;
   }
 
-  explicit StickyMarkSweep(Heap* heap, bool is_concurrent, const std::string& name_prefix = "");
+  StickyMarkSweep(Heap* heap, bool is_concurrent, const std::string& name_prefix = "");
   ~StickyMarkSweep() {}
 
  protected: