Remove config for Performance/Casecmp Rubocop
Remove config for Performance/Casecmp Rubocop

- As of govuk-lint 3.11.2, govuk-lint doesn't include Performance
  Rubocops because they were split out from the core Rubocop gem.
- This config here causes warnings in `govuk-lint-ruby` output because
  we don't require 'rubocop-performance' to get these Performance cops:
  `Warning: unrecognized cop Performance/Casecmp found in .rubocop.yml`
- Given that the Performance cops were removed so many versions of
  govuk-lint ago, I'm going to take a punt that we don't need them in
  this app and so we can get rid of this line to get rid of three (!)
  warnings.

alphagov/transition,alphagov/transition,alphagov/transition