Stop running `pact:verify:branch` as part of standard build
Stop running `pact:verify:branch` as part of standard build

We already run `pact:verify` in the default rake task:
https://github.com/alphagov/frontend/blob/693146acc420b58e83aaa8a99f864b18d8f8f9a3/Rakefile#L16

We don't need to explicitly run it _again_.

But we do need to _keep_ the `pact:verify:branch` task, as it is
called by gds-api-adapters:
https://github.com/alphagov/gds-api-adapters/blob/67ad7bece4b3a988e989ef90c4dde22871b5794b/Jenkinsfile#L76-L85

We _could_ delete it if we made more use of `PACT_URI`, but this
makes for a more complex and brittle experience, so on balance is
not worth it:
https://github.com/alphagov/frontend/pull/2647

alphagov/frontend,alphagov/frontend,alphagov/frontend,alphagov/frontend