Remove unnecessary `explicit` qualifiers on constructors.

Change-Id: Id12e392ad50f66a6e2251a68662b7959315dc567
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: