Cleanup: VSE: Remove StripData.start/endstill

Not to be confused with `Strip.start/endstill` which were renamed with
a legacy suffix. The `StripData` version here isn't present anywhere
in versioning code or otherwise, so it should be safely removable.

Reduces the size of this struct by 8 bytes.

Pull Request: https://projects.blender.org/blender/blender/pulls/143874

source/blender/makesdna/DNA_sequence_types.h

@@ -125,7 +125,6 @@ typedef struct StripProxy {
 typedef struct StripData {
   struct StripData *next, *prev;
   int us, done;
-  int startstill, endstill;
   /**
    * Only used as an array in IMAGE sequences(!),
    * and as a 1-element array in MOVIE sequences,
@@ -467,6 +466,7 @@ typedef struct TextVars {
 
   /** Replaced by `anchor_y` in 4.4. */
   char align_y_legacy DNA_DEPRECATED; /* eEffectTextAlignY */
+
   char anchor_x; /* eEffectTextAlignX */
   char anchor_y; /* eEffectTextAlignY */
   char _pad1;