apache: remove $rejected_pkgs

- On precise, the module is just a stub and doesn't have anything to do
  with the real work of apache. The version of apache installed will be
  decided by which mpm you chose, and it will automatically remove upon
  installation all other mpms. If config is not badly broken by us, the
  OS will take care of this itself.

- On trusty, removing the module config cannot depend on removing the
  mpm packages, which are just transitional and empty.

Change-Id: I319adfb5769688da802125d3731d609d47b13eb2
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>