Closes #48256

It appears that the current git remotes support was implemented in
#42819, following the design described in
https://github.com/zed-industries/zed/pull/42486#issuecomment-3524092306.
That design does not include an interaction for deleting remotes, but
the delete remote branch action was mistakenly implemented as deleting
the remote itself.

After this PR, there should be no code paths that use `remove_remote`
anymore. I've kept it for now though, as it may be useful when we
introduce the corresponding interaction in the future.

Release Notes:

- Fixed a bug where deleting a remote branch from the branch picker
would incorrectly remove the entire remote configuration