GitTfsRemote: remove unused variable to silence build warning
GitTfsRemote: remove unused variable to silence build warning

In 8eca32f4 (GitTfsRemote: Cleanup gitignore support configuration handling,
2018-08-11) the last remaining user of the _disableGitingoreSupport variable
was removed, but the member variable was not. Remove it now to silence
a build warning.

PKRoma/git-tfs,git-tfs/git-tfs