debian/rules: remove remnants of git-core package

debian/rules: remove remnants of git-core package

The Git Debian package fails to build with dpkg-buildpackage twice in
a row.  The first build succeeds, but the second one fails:

   debian/rules override_dh_installdocs-indep
  make[1]: Entering directory '/build/git-2.15.1+next.20171214'
  install -d -m0755 '/build/git-2.15.1+next.20171214/debian/git'-core/usr/share/doc
  ln -s git '/build/git-2.15.1+next.20171214/debian/git'-core/usr/share/doc/git-core
  ln: failed to create symbolic link '/build/git-2.15.1+next.20171214/debian/git-core/usr/share/doc/git-core': File exists
  debian/rules:154: recipe for target 'override_dh_installdocs-indep' failed

That's because the git-core package is no longer being built, so
dh_clean does not clean up the debian/git-core/ tree, but the rules to
generate that tree remain as lingering cruft.

These rules should have been removed in 6e56d199 (debian: remove
transitional git-core package, 2017-10-10).  Finish up by removing
them now.

Fixes: https://bugs.debian.org/884890
Reported-by: Andreas Beckmann <anbe@debian.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>