Cleanup: animation, remove BONE_UNKEYED flag

Remove the `BONE_UNKEYED` flag. It was only written (set/cleared) but
never actually read.

Also remove `framechange_poses_clear_unkeyed()` as its only function was
to clear the `BONE_UNKEYED` flag. It wasn't used anywhere either.

The only code that used the flag was the `extract_pose_from_action()`,
which was removed in 2869ce6cfa (2009).

No functional changes.

source/blender/editors/animation/keyframing.c

@@ -3034,19 +3034,9 @@ bool ED_autokeyframe_pchan(
     ANIM_apply_keyingset(C, &dsources, NULL, ks, MODIFYKEY_MODE_INSERT, (float)CFRA);
     BLI_freelistN(&dsources);
 
-    /* clear any unkeyed tags */
-    if (pchan->bone) {
-      pchan->bone->flag &= ~BONE_UNKEYED;
-    }
-
     return true;
   }
 
-  /* add unkeyed tags */
-  if (pchan->bone) {
-    pchan->bone->flag |= BONE_UNKEYED;
-  }
-
   return false;
 }