Remove `git_process` metrics probes

What does this MR do?

Implements:
gitlab#290765 (closed) from
&5035

probe_git method is implemented in GitProcessProber (https://gitlab.com/gitlab-org/gitlab-exporter/-/blob/master/lib/gitlab_exporter/git.rb#L93), which, in turn, uses regular ProcessProber (https://gitlab.com/gitlab-org/gitlab-exporter/-/blob/master/lib/gitlab_exporter/git.rb#L108).

ProcessProber produces metrics with process_* prefix (https://gitlab.com/gitlab-org/gitlab-exporter/-/blob/master/lib/gitlab_exporter/process.rb#L73), and it was discussed in gitlab#290765 (closed), that it should be safe to remove them.

According to what Matthias found (gitlab#290765 (comment 461467963)), the only git-related thing could be git_upload_pack, which was discussed to be safe for removal (gitlab#290765 (comment 468990598)).

In general, all Git-related metrics are expected to be exported by Gitaly for consistency.

⚠ Note: this doesn't touch metrics produced by GitProber (https://gitlab.com/gitlab-org/gitlab-exporter/-/blob/master/lib/gitlab_exporter/git.rb#L64), such as git_pull_time_milliseconds, we remove only what is produced by GitProcessProber.

Checklist

See Definition of done.

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Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
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Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
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• Equivalent MR/issue for the GitLab Chart opened