Remove perl library from Ubuntu installs

It seems that the package was added a long time ago [1], and might
have been needed for the percona tooling. This makes sense if we're
downloading the dep - but we're no longer doing that and can rely
on apt's resolver to find the correct packages.

[1] https://review.openstack.org/238748

Change-Id: I742c4ef9a201384787d5a9aeafac4ff7b0bdc353

vars/ubuntu-16.04.yml

@@ -35,7 +35,6 @@ galera_server_required_distro_packages:
   - debconf-utils
   - libaio1
   - libc6
-  - libdbd-mysql-perl
   - libgcc1
   - libgcrypt20
   - libstdc++6