pakfire: Replace getmetadata duplicate code

pakfire: Replace getmetadata duplicate code

- Use getmetadata function in services.cgi to determine installed
  addon services to display. Removing code duplication and intel that
  should only be known by pakfire itself.
- Removed hardcoded exclusions:
  - squid should show up correctly using the new metadata info
  - mdadm is part of core and will never show up here
  - alsa, unknown if this problem still exists, but if it is, this
    should be handled somewhere else.

Signed-off-by: Robin Roevens <robin.roevens@disroot.org>