debian: remove git-el package

debian: remove git-el package

Since 6d5ed4836d (git{,-blame}.el: remove old bitrotting Emacs code,
2018-04-11), the emacs modules in the git-el package have consisted
only of an "(error)" call to help people find a more modern,
maintained module.  Alas, packaging even for that has proven
difficult to maintain, producing errors like

  /usr/lib/emacsen-common/packages/install/git emacs xemacs21 failed at /usr/lib/emacsen-common/lib.pl line 19, <TSORT> line 1.

Remove the compatibility package, with a Breaks to ensure that
people's installations get cleaned up.

Fixes: https://bugs.debian.org/987264
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>