[Hart] remove unneeded base_url override

Bromley (from which Hart cobrand code was templated) presumably needed
some customization in base_url, however the default behaviour from
::UKCouncils gives the correct behaviour for Hart.

(and in this case, the unnecessary customization prevented the two_tier
urls from being resolved correctly.)

This allows the inherited method to Do The Right Thing with
redirects to problems logged for the surrounding authority
(Hampshire in this case)