Remove unused gettext_i18n_rails_js gem

What does this MR do and why?

This gem was mainly used to compile PO files to JSON. This usage has been removed with:

• !115042 (merged)

JiHu seems to have adopted this with:

• https://jihulab.com/gitlab-cn/gitlab/-/merge_requests/1614

So we can safely remove the gem.

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.