d/rules: Remove remark about libapt-pkg-perl

The warning for reminding people to run:
  $ debian/rules refresh-perl-provides

includes a reminders to install libapt-pkg-perl.  However, since
lintian Depeds (and Build-Depends) on libapt-pkg-perl, the odds of
getting that far in the rules file without having libapt-pkg-perl
installed is slim to none.

Signed-off-by: Niels Thykier <niels@thykier.net>