Remove fetch_managed_component_version feature flag

When releasing GitLab via the ReleaseTools::PublicRelease::GitlabRelease class, we currently make a request to fetch the VERSION file contents of each component enrolled in managed versioning. As mentioned in gitlab-org/release-tools!2086 (comment 1205544652), this is an unnecessary API call since the version returned by the API call should be the same as the GitLab version being released.

gitlab-org/release-tools!2086 (merged) added a logger to log warnings if the versions ever do not match. Assuming no warnings are logged, this feature flag and the enclosed API calls can be safely removed after 2023-03-07 (tentative date just over 3 major releases after the warning logging has taken effect).