Drop unneeded progress reporting

When pulling the ostree-metadata branch for p2p remotes we passed
NULL for progress which caused it to report with the default console
progress, which doesn't make any sense for the (small) ostree-metadata
branch.

Closes: #2456
Approved by: alexlarsson
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f