mapi: THREADS was always defined, remove it

THREADS was defined if HAVE_PTHREADS or _WIN32 was defined.  That's
always the case.  The build would die in c11/threads.h otherwise.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>

src/mapi/glapi/glapi_priv.h

@@ -86,10 +86,8 @@ get_entrypoint_address(unsigned int functionOffset);
 #if defined(USE_X86_ASM)
 # if defined(GLX_USE_TLS)
 #  define DISPATCH_FUNCTION_SIZE  16
-# elif defined(THREADS)
-#  define DISPATCH_FUNCTION_SIZE  32
 # else
-#  define DISPATCH_FUNCTION_SIZE  16
+#  define DISPATCH_FUNCTION_SIZE  32
 # endif
 #endif