fixed build with g++ 5+ and ASAN enabled

Summary:
Fixing by deleting unnecessary macro checks - folly already requires at least g++4.8
Closes https://github.com/facebook/folly/pull/338

Reviewed By: yfeldblum

Differential Revision: D2742024

Pulled By: fredemmott

fb-gh-sync-id: c0ecc7edceeebd1f972b0be4f4740214211f1c24

folly/FBString.h

@@ -119,8 +119,6 @@ namespace folly {
 #  endif
 # endif
 #elif defined (__GNUC__) && \
-      (__GNUC__ == 4) && \
-      (__GNUC_MINOR__ >= 8) && \
       __SANITIZE_ADDRESS__
 # define FBSTRING_DISABLE_ADDRESS_SANITIZER \
     __attribute__((__no_address_safety_analysis__, __noinline__))