cpplint: Cleanup errors

Cleanup errors from upstream cpplint in preparation
for moving art's cpplint fork to upstream tip-of-tree cpplint.

Test: cd art && mm
Bug: 68951293
Change-Id: I15faed4594cbcb8399850f8bdee39d42c0c5b956
diff --git a/runtime/gc/reference_processor.h b/runtime/gc/reference_processor.h
index a8135d9..1d984eb 100644
--- a/runtime/gc/reference_processor.h
+++ b/runtime/gc/reference_processor.h
@@ -45,7 +45,7 @@
 // Used to process java.lang.ref.Reference instances concurrently or paused.
 class ReferenceProcessor {
  public:
-  explicit ReferenceProcessor();
+  ReferenceProcessor();
   void ProcessReferences(bool concurrent,
                          TimingLogger* timings,
                          bool clear_soft_references,