remove repository.callsign from .arcconfig

[freebsd_igalic.co]

Summary

The current set of git repositories in our Phabricator doesn't have any
call-signs, they are still associated with the old Subversion
repositories.

Removing the callsign allows arc to correctly determine the repository.

Test Plan

run arc which without the patch, result:

This working copy is associated with the FreeBSD src repository - subversion repository.

run arch which with this patch:

This working copy is associated with the FreeBSD src repository repository.

[lwhsu]

In D27972, we're testing to reuse this callsign for git repository.