def remove(self, email):
        """Remove a Collaborator.

        Args:
            str : Collaborator email address.
        """
        if email in self._collaborators:
            if self._collaborators[email] == ShareRequestValue.Add:
                del self._collaborators[email]
            else:
                self._collaborators[email] = ShareRequestValue.Remove
        self._dirty = True
Remove a Collaborator.

        Args:
            str : Collaborator email address.