gmtime: remove define

It turns out some systems rely on the gmtime or gmtime_r to be defined
already in the system headers and thus my "precaution" redefining of
them only caused trouble. They are now removed.

lib/parsedate.c

@@ -529,9 +529,9 @@ CURLcode Curl_gmtime(time_t intime, struct tm *store)
   const struct tm *tm;
 #ifdef HAVE_GMTIME_R
   /* thread-safe version */
-  tm = (struct tm *)(gmtime_r)(&intime, store);
+  tm = (struct tm *)gmtime_r(&intime, store);
 #else
-  tm = (gmtime)(&intime);
+  tm = gmtime(&intime);
   if(tm)
     *store = *tm; /* copy the pointed struct to the local copy */
 #endif