remove add_namespace call for project creation

What does this MR do?

Original MR: gitlab-foss!32258 (closed)

Refactor Gitlab::Shell a bit in preparation of the removal of AddNamespace. We don't need to call AddNamespace when we create a repository because the Gitaly RPC does a mkdir -p when creating a repository: https://gitlab.com/gitlab-org/gitaly/blob/master/internal%2Fservice%2Frepository%2Fcreate.go#L18

Part of: gitaly#1891 (closed)

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Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
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Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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