leveldb: Remove *_unlocked feature detection from POSIX port.

CL 170738066 removed all instances of fread_unlocked, fwrite_unlocked
and fflush_unlocked calls from the codebase, so the feature detection
can be removed as well.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=171154269

port/port_posix.h

@@ -50,15 +50,6 @@
 #define PLATFORM_IS_LITTLE_ENDIAN (__BYTE_ORDER == __LITTLE_ENDIAN)
 #endif
 
-#if defined(__APPLE__) || defined(OS_SOLARIS) || defined(OS_FREEBSD) ||\
-    defined(OS_NETBSD) || defined(OS_OPENBSD) || defined(OS_DRAGONFLYBSD) ||\
-    defined(OS_ANDROID) || defined(OS_HPUX) || defined(CYGWIN)
-// Use fread/fwrite/fflush on platforms without _unlocked variants
-#define fread_unlocked fread
-#define fwrite_unlocked fwrite
-#define fflush_unlocked fflush
-#endif
-
 #if defined(__APPLE__) || defined(OS_FREEBSD) ||\
     defined(OS_OPENBSD) || defined(OS_DRAGONFLYBSD)
 // Use fsync() on platforms without fdatasync()