Cleanup: Remove scene frame macros (CFRA et al.)

Removes the following macros for scene/render frame values:
- `CFRA`
- `SUBFRA`
- `SFRA`
- `EFRA`

These macros don't add much, other than saving a few characters when typing.
It's not immediately clear what they refer to, they just hide what they
actually access. Just be explicit and clear about that.
Plus these macros gave read and write access to the variables, so eyesores like
this would be done (eyesore because it looks like assigning to a constant):
```
CFRA = some_frame_nbr;
```

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D15311

source/blender/sequencer/intern/strip_relations.c

@@ -250,7 +250,7 @@ void SEQ_relations_free_imbuf(Scene *scene, ListBase *seqbase, bool for_render)
   SEQ_prefetch_stop(scene);
 
   for (seq = seqbase->first; seq; seq = seq->next) {
-    if (for_render && SEQ_time_strip_intersects_frame(scene, seq, CFRA)) {
+    if (for_render && SEQ_time_strip_intersects_frame(scene, seq, scene->r.cfra)) {
       continue;
     }