Remove undeeded assert.

This breaks compatibility with some legacy over the air mpegts content on US feeds.

src/clock/clock.c

@@ -995,7 +995,6 @@ vlc_clock_t *vlc_clock_main_CreateMaster(vlc_clock_main_t *main_clock,
 
 vlc_clock_t *vlc_clock_main_CreateInputMaster(vlc_clock_main_t *main_clock)
 {
-    vlc_mutex_assert(&main_clock->lock);
     struct vlc_clock_context *ctx = main_clock->context;
 
     /* The master has always the 0 priority */