Remove router rake task

This is a hangover of the old router. Trade Tariff doesn't register against
the new router (which is a good thing because it wouldn't work as it would
register in preview with a domain of production.alphagov.co.uk because it's
based on FACTER_govuk_platform)

I'm removing FACTER_govuk_platform and I stumbled across this, so I thought
I'd just delete the rake task.

This fixes #131