GIT URL should not be a deprecated setting (TNL-396).

Do not deprecate giturl.
Issue: https://openedx.atlassian.net/browse/TNL-396

Acceptance criteria: 
* If feature flag ENABLE_EXPORT_GIT is off, don't show the setting at all on the Advanced Settings page. 
* If feature flag ENABLE_EXPORT_GIT is on, show the setting as a non-deprecated Advanced Setting. 

Reviewers: @auraz @tymofij 
The only thing that "deprecated" impacts is whether or not it is marked as deprecated in Advanced Settings. So no, don't need to worry about migration problems.