contrib: remove remote-helper stubs

The "remote-helpers" directory contains two remote helper scripts for
Mercurial and Bazaar. These scripts have since been converted into stubs
in b2c851a (Revert "Merge branch 'jc/graduate-remote-hg-bzr' (early
part)", 2014-05-20) as the helpers have been moved into their own
upstream projects [1][2].

Given that these stubs have been created more than a decade ago it is
very unlikely that anybody still tries to use them. Remove them.

[1]: https://github.com/felipec/git-remote-bzr
[1]: https://github.com/felipec/git-remote-hg

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t