Remove onboarding git_write_at column

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.

Remove the git_write_at column, as it is not used anymore because the created_at is used now.