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 
@cahrens I thought about it previously. But the methods that are all classmethods and also there is copying of the filtered list to avoid permanently changing the class attribute that is present stopped doing me via init. Anyway I updated code with another solution that can be used for other fields depending on feature flags. Please take a look.