debian: fix ceph-mgr-modules-core files

debian: fix ceph-mgr-modules-core files

Remove the files from ceph-mgr that are added to ceph-mgr-modules-core.

2020-02-21T17:59:56.031 INFO:teuthology.orchestra.run.smithi060.stdout:dpkg: error processing archive /tmp/apt-dpkg-install-YpwLHV/044-ceph-mgr_15.1.0-1037-ga6b324b-1bionic_amd64.deb (--unpack):
2020-02-21T17:59:56.031 INFO:teuthology.orchestra.run.smithi060.stdout: trying to overwrite '/usr/share/ceph/mgr/alerts/__init__.py', which is also in package ceph-mgr-modules-core 15.1.0-1037-ga6b324b-1bionic

Broken by 589626464d475b4ad96905fbddf11a5782a040f0

Signed-off-by: Sage Weil <sage@redhat.com>