release build treat warnings as errors

Summary:
fixing warnings is important, especially for release code.
Closes https://github.com/facebook/rocksdb/pull/2971

Differential Revision: D5980596

Pulled By: ajkr

fbshipit-source-id: 04f4ea3fb005dcda33d60342e4361e380bc4dfb1

Makefile

@@ -108,11 +108,9 @@ CFLAGS +=  -DHAVE_POWER8
 HAVE_POWER8=1
 endif
 
-# if we're compiling for release, compile without debug code (-DNDEBUG) and
+# if we're compiling for release, compile without debug code (-DNDEBUG)
-# don't treat warnings as errors
 ifeq ($(DEBUG_LEVEL),0)
 OPT += -DNDEBUG
-DISABLE_WARNING_AS_ERROR=1
 
 ifneq ($(USE_RTTI), 1)
 	CXXFLAGS += -fno-rtti