d/postrm: avoid purging/masking networking.service

d/postrm: avoid purging/masking networking.service

Proxmox projects are expecting one of ifupdown or ifupdown2 anyway,
so the networking.service must always exist. This makes switching
from ifupdown to ifupdown2 more convenient.

Note that it doesn't fixes the systems that already switched, but
still have a lingering ifupdown conf available, They won't get this
update, and if they do a (config) purge of the (partially) removed
ifupdown it will still disable networking.service - we need to handle
that somewhere else (e.g., sed the ifupdown postrm in ifupdown2 preinst)

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>