Remove experimental code for 'readme_from_gitaly' FF

What does this MR do and why?

Contributes to #387703 (closed)

Unfortunately, the proposed solution was not performant enough during testing. SearchFilesByName requests cannot be cached and worsen the performance instead of improving it.

I remove the feature flag and new experimental code, in favor of a different solution.

This commit reverts MRs: !109209 (merged) and !108609 (merged)

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