Remove PLATFORM_IS_LITTLE_ENDIAN from port/posix.h.

This is an accidental leftover from the CMake migration. The macro has
been replaced with LEVELDB_IS_BIG_ENDIAN.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=192364918

port/port_posix.h

@@ -36,10 +36,6 @@
 #include "port/atomic_pointer.h"
 #include "port/thread_annotations.h"
 
-#ifndef PLATFORM_IS_LITTLE_ENDIAN
-#define PLATFORM_IS_LITTLE_ENDIAN (__BYTE_ORDER == __LITTLE_ENDIAN)
-#endif
-
 #if !HAVE_FDATASYNC
 #define fdatasync fsync
 #endif  // !HAVE_FDATASYNC