Remove `stage_button.vue` file

Remove stage_button.vue file

The following discussion from !21902 (merged) should be addressed:

• @himkp started a discussion: (+1 comment)

  Note that this modal was present in app/assets/javascripts/ide/components/commit_sidebar/stage_button.vue, but that component itself was removed from view in %11.4 in gitlab-foss!21845 (diffs), hence causing a regression.

  I've added a new modal to this file and not removed the modal in stage_button.vue. As a cleanup task, we can probably remove stage_button file altogether.