Use ATTRIBUTE_UNUSED more.

Use it in lieu of UNUSED(), which had some incorrect uses.

Change-Id: If247dce58b72056f6eea84968e7196f0b5bef4da
diff --git a/runtime/debugger.cc b/runtime/debugger.cc
index a4f95b6..b17b76e 100644
--- a/runtime/debugger.cc
+++ b/runtime/debugger.cc
@@ -1903,8 +1903,7 @@
 JDWP::JdwpError Dbg::GetThreadName(JDWP::ObjectId thread_id, std::string* name) {
   ScopedObjectAccessUnchecked soa(Thread::Current());
   JDWP::JdwpError error;
-  Thread* thread = DecodeThread(soa, thread_id, &error);
-  UNUSED(thread);
+  DecodeThread(soa, thread_id, &error);
   if (error != JDWP::ERR_NONE && error != JDWP::ERR_THREAD_NOT_ALIVE) {
     return error;
   }
@@ -1931,8 +1930,7 @@
   }
   ScopedAssertNoThreadSuspension ants(soa.Self(), "Debugger: GetThreadGroup");
   // Okay, so it's an object, but is it actually a thread?
-  Thread* thread = DecodeThread(soa, thread_id, &error);
-  UNUSED(thread);
+  DecodeThread(soa, thread_id, &error);
   if (error == JDWP::ERR_THREAD_NOT_ALIVE) {
     // Zombie threads are in the null group.
     expandBufAddObjectId(pReply, JDWP::ObjectId(0));