Remove "DifferentialGitSVNIDField" custom field in Differential

Summary: Ref T11114. This is obsolted by the narrower DifferentialGitSVNIDCommitMessageField.

Test Plan: Browsed around.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17075