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 
I've not found this property in https://github.com/edx/XBlock/blob/81a6d713c98d4914af96a0ca624ee7fa4903625e/xblock/fields.py.
I just hope there will be no migration problems here.