Delete Unused MR Widget size_limit_reached Partial

What does this MR do and why?

The ee/app/views/projects/merge_requests/widget/open/_size_limit_reached.html.haml partial does not appear to be used.

I cannot find any reference to it in the code.

It looks like the file was introduced here under the app/views (non-ee) folder where it was used in app/views/projects/merge_requests/widget/_open.html.haml.

app/views/projects/merge_requests/widget/_open.html.haml was then deleted here.

But _size_limit_reached.html.haml still existed at that point: https://gitlab.com/gitlab-org/gitlab/-/tree/7d157a8ab45617e35f097d85733ed9cf76488977/app/views/projects/merge_requests/widget/open

If I check out that commit, 7d157a8ab45617e35f097d85733ed9cf76488977, I cannot find any reference to the file in the code. It probably should have been deleted with 7d157a8a.

_size_limit_warning was then moved to its current ee directory here: 507353bc

Here's the history of the file: https://gitlab.com/gitlab-org/gitlab/-/commits/master/ee/app/views/projects/merge_requests/widget/open/_size_limit_reached.html.haml

Discovered while working on https://gitlab.com/gitlab-org/gitlab/-/issues/354363.

See also notes here and here.

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