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) {
   }