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/editors/io/io_alembic.c

@@ -144,10 +144,10 @@ static int wm_alembic_export_exec(bContext *C, wmOperator *op)
   /* Take some defaults from the scene, if not specified explicitly. */
   Scene *scene = CTX_data_scene(C);
   if (params.frame_start == INT_MIN) {
-    params.frame_start = SFRA;
+    params.frame_start = scene->r.sfra;
   }
   if (params.frame_end == INT_MIN) {
-    params.frame_end = EFRA;
+    params.frame_end = scene->r.efra;
   }
 
   const bool as_background_job = RNA_boolean_get(op->ptr, "as_background_job");
@@ -248,8 +248,8 @@ static void wm_alembic_export_draw(bContext *C, wmOperator *op)
   Scene *scene = CTX_data_scene(C);
 
   if (scene != NULL && RNA_boolean_get(op->ptr, "init_scene_frame_range")) {
-    RNA_int_set(op->ptr, "start", SFRA);
-    RNA_int_set(op->ptr, "end", EFRA);
+    RNA_int_set(op->ptr, "start", scene->r.sfra);
+    RNA_int_set(op->ptr, "end", scene->r.efra);
 
     RNA_boolean_set(op->ptr, "init_scene_frame_range", false);
   }