Remove check for correct core_plugin when using ml2.

This checks fails, because $::neutron::core_plugin
is not available in the context of ml2.pp.

Change-Id: I96c11c5a57c530f72e0e71447831b8219c12a685
(cherry picked from commit 2cc78fb7850c913afb5e79aa0eef24f7f6a92ba0)