debian: drop unused deps from ceph-mgr-modules-core.requires

CherryPy and packaging are listed as runtime dependencies of
ceph-mgr-modules-core, but none of the modules shipped in that package
import either one. CherryPy is used by the dashboard, cephadm, and
prometheus modules, which list it in their own .requires; packaging is
not imported by any shipped mgr code. Both entries therefore pull
python3-cherrypy and python3-packaging into ceph-mgr-modules-core for no
reason.

Drop them. The remaining entries (natsort, requests, python-dateutil,
pyyaml) match what the core modules import; prettytable is declared
separately in debian/control.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>