gmain: remove unnecessary initialization of *timeout in prepare() callbacks

Note that the prepare callback only has one caller, which pre-initializes
the timeout argument to -1. That may be an implementation detail and not
publicly promised, but it wouldn't make sense to do it any other way in
the caller.

Also, note that g_unix_signal_watch_prepare() and the UNIX branch of
g_child_watch_prepare() already relied on that.

gio/gsocket.c

@@ -4003,8 +4003,6 @@ socket_source_prepare (GSource *source,
 {
   GSocketSource *socket_source = (GSocketSource *)source;
 
-  *timeout = -1;
-
 #ifdef G_OS_WIN32
   if ((socket_source->pollfd.revents & G_IO_NVAL) != 0)
     return TRUE;