Revert "prevent #1312 from occuring (refs #1312)"

This reverts commit 69d572ca.
It's not an issue with sout-keep alone. The issue shows up if using
both sout-keep and #gather, and there is no easy way for a stream
object to know what its upstream stream (if any) is. Hence, let users
shoot themselves in the feet if they want, and otherwise assume that
people don't use #gather _in_front_ of #display.

modules/stream_out/display.c

@@ -96,13 +96,6 @@ static int Open( vlc_object_t *p_this )
     sout_stream_t     *p_stream = (sout_stream_t*)p_this;
     sout_stream_sys_t *p_sys;
 
-    /* FIXME: this sucks big time (see #1312) */
-    if( var_CreateGetBool( p_stream, "sout-keep" ) )
-    {
-        msg_Err( p_stream, "cannot use #display sout with sout-keep option" );
-        return VLC_EGENERIC;
-    }
-
     p_sys = malloc( sizeof( sout_stream_sys_t ) );
     if( p_sys == NULL )
         return VLC_ENOMEM;