Don't try to enable mod_perl upon libembperl-perl installation

When libapache2-mod-perl2 is being installed, it enables mod_perl by
default. This also is the case if pulled in by libembperl-perl via
Recommends.

So if libapache2-mod-perl2 but not enabled, that's a local
administrator decision which should be honoured (and actually is
honoured by apache2_invoke, so the removed lines were defacto a no-op
anyways).

Closes: #810655 (properly and with minimal changes)