Drop support for GCC < 4.8 in folly/FBString.h

Summary: [Folly] Drop support for GCC < 4.8 in `folly/FBString.h`.

Can't stay indefinitely backcompat with older compilers.

Reviewed By: @JoelMarcey

Differential Revision: D2441747

folly/FBString.h

@@ -255,15 +255,6 @@ private:
 };
 */
 
-/**
- * gcc-4.7 throws what appears to be some false positive uninitialized
- * warnings for the members of the MediumLarge struct.  So, mute them here.
- */
-#if defined(__GNUC__) && !defined(__clang__)
-# pragma GCC diagnostic push
-# pragma GCC diagnostic ignored "-Wuninitialized"
-#endif
-
 /**
  * This is the core of the string. The code should work on 32- and
  * 64-bit and both big- and little-endianan architectures with any
@@ -899,10 +890,6 @@ private:
   }
 };
 
-#if defined(__GNUC__) && !defined(__clang__)
-# pragma GCC diagnostic pop
-#endif
-
 #ifndef _LIBSTDCXX_FBSTRING
 /**
  * Dummy fbstring core that uses an actual std::string. This doesn't