Deprecated (undocumented) g_thread_gettime

g_thread_gettime() is an undocumented public function pointer that
points to a function that returns the monotonic time in nanoseconds.
g_get_monotonic_time() does the same in microseconds, so it can be used
instead.

GLib had one internal user in GFileMonitor that only cared about
millisecond accuracy; it has been ported to g_get_monotonic_time().

gthread/gthread-impl.c

@@ -315,8 +315,6 @@ g_thread_init (GThreadFunctions* init)
     g_thread_use_default_impl = FALSE;
 
   g_thread_functions_for_glib_use = *init;
-  if (g_thread_gettime_impl)
-    g_thread_gettime = g_thread_gettime_impl;
 
   supported = (init->mutex_new &&
 	       init->mutex_lock &&