Remove old feature-trailblock route from dev-build routes file

CC @gklopper

I think that this feature was removed in [4be38b4e2e2943eb39b0a3989a29cc87459e331e](https://github.com/guardian/frontend/commit/4be38b4e2e2943eb39b0a3989a29cc87459e331e) but the route for it still remains in the `dev-build` `dev-build/conf/routes` file.

This causes a compilation error on `dev-build` in SBT; removing it fixes the error.

Edit:
Credit for solving this goes to @rtyley