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 
Instead of having to put the same code 3 times, it would be nicer if 'giturl' was just added to FILTERED_LIST based on whether or not the setting exists. I'm not sure if there is a way to do that with the present declaration of FILTERED_LIST, but couldn't it be done in an __init__ method?