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_entrypoint.c

@@ -120,11 +120,9 @@ fill_in_entrypoint_offset(_glapi_proc entrypoint, unsigned int offset)
 
 #if defined(GLX_USE_TLS)
    *((unsigned int *)(code +  8)) = 4 * offset;
-#elif defined(THREADS)
+#else
    *((unsigned int *)(code + 11)) = 4 * offset;
    *((unsigned int *)(code + 22)) = 4 * offset;
-#else
-   *((unsigned int *)(code +  7)) = 4 * offset;
 #endif
 }