Remove duplicate git_write from the post receive git hook

Currently we have an onboarding_progresses table column called git_write.

This issue is meant to drive the correct placement of the recording of this Learn GitLab task as right now it is in the git post receive hook, which is fired on successful git writes, which would make sense if we were tracking code adding for Learn GitLab.

However, we have that mapped to Create a repository in Learn Gitlab.

Therefore if this is to track repository creation, we should:

• Remove the placement of this action in post receive as it is being put in the right place in !149617 (merged) to address a 'apply template' not recording this issue.
  • handled by Move all uses of Onboarding::ProgressService to ee (#472657 - closed)
• Update namespaces who may have repositories already created, but didn't actually 'write' to trigger this yet.
  • this is a nice to have. Check with PM on if we really need to do this.
• Create a follow-up to rename this column from git_write_at to something more sensible like repository_created_at.
  • handled by Remove onboarding git_write_at column (#487919 - closed)
• Replace the git_write_at column with the created_at column.
  • done in scope of this issue.
• Remove the git_write_at column, as not used anymore.
  • handled by Remove onboarding git_write_at column (#487919 - closed)

original description

The following discussion from !149617 (merged) should be addressed:

• @dstull started a discussion:

  note (non-blocking):

  Follow-up here to determine if we can remove this fully and migrate data if needed.