gitaly: Stop setting Rugged's gitconfig search path

What does this MR do?

In the past, Gitaly required administrators to configure Rugged's gitconfig search path to point to the gitconfig that's also used by the Git command line as spawned by Gitaly. This configuration really only needed to contain a single config entry core.fsyncObjectFiles=true, which is mandatory to avoid causing repository corruption. All the other config entries that can be set in there don't have any effect nowadays anymore given that Gitaly's Ruby sidecar isn't doing a lot anymore.

Given that the configuration must really only contain a single fixed key, Gitaly has introduced logic to just write its own gitconfig for Rugged in case the Rugged search path isn't set by the administrator. Consequentially, there is no need for us to set it anymore, and Gitaly will eventually remove the configuration altogether.

So let's stop setting the Rugged gitconfig search path and instead use Gitaly's default configuration.

Related issues

• Omnibus MR removing this setting: gitlab-org/omnibus-gitlab!6105 (merged)
• Gitaly epic to deprecate reading external gitconfig: gitlab-org&8069 (closed)
• Gitaly issue that enables fsyncing in case Rugged search path is unset: gitlab-org/gitaly#4232 (closed)
• Gitaly issue to track deprecation of Rugged search path config: gitlab-org/gitaly#4233 (closed)

Checklist

See Definition of done.

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Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened