Fix [#22256] bpy.ops.sequencer.delete.poll() not working

Just about all sequencer operator poll functions were requiring an active 
sequence editor space type. This wasn't necessary for most of them, and prevented use 
from scripts, console, etc.

source/blender/editors/space_sequencer/sequencer_intern.h

@@ -67,6 +67,9 @@ int event_to_efftype(int event);
 int seq_effect_find_selected(struct Scene *scene, struct Sequence *activeseq, int type, struct Sequence **selseq1, struct Sequence **selseq2, struct Sequence **selseq3, char **error_str);
 struct Sequence *alloc_sequence(struct ListBase *lb, int cfra, int machine);
 
+/* operator helpers */
+int sequencer_edit_poll(struct bContext *C);
+
 /* externs */
 extern EnumPropertyItem sequencer_prop_effect_types[];
 extern EnumPropertyItem prop_side_types[];