Unpin file from file browser
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The following discussion from !137544 should be addressed:
- [ ] @danmh started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/137544#note_1683735250):
> **suggestion** (non-blocking): Consider allowing the file to be unpinned from the file browser
>
> {width="158" height="269"}
>
> _(**note:** my concerns with the pin metaphor are documented in https://gitlab.com/gitlab-org/gitlab/-/issues/433250#note_1683732675 but I think out of scope of this MR.)_
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