Sequencer: free cache used by sequence when removing strip

Before this removing and adding new strip could have been lead
into situations when new sequence would use old cache.

source/blender/blenkernel/intern/sequencer.c

@@ -136,6 +136,8 @@ static void free_proxy_seq(Sequence *seq)
 		IMB_free_anim(seq->strip->proxy->anim);
 		seq->strip->proxy->anim = NULL;
 	}
+
+	BKE_sequencer_cache_cleanup_sequence(seq);
 }
 
 static void seq_free_strip(Strip *strip)