[Feature flag] Removal of gitaly_git_v250

What

Remove the :gitaly_git_v250 feature flag.

Owners

• Team: Git
• Most appropriate slack channel to reach out to: #g_git
• Best individual to reach out to: @justintobler

Steps

• Remove the feature flag and the pre-feature-flag code (howto)
• Wait for the MR to be deployed to production
• Remove the feature flag via chatops (howto)

Please refer to the documentation of feature flags for further information.