Remove `determine_blank_ref_based_on_gitaly_object_format` feature flag

This commit removes the
`determine_blank_ref_based_on_gitaly_object_format` feature flag to
ensure we always determine a repository blank sha reference based on
gitaly's `ObjectFormatRequest` which determines a repository's object
format (i.e. whether it's `sha1` or `sha256`).

Changelog: changed