Remove `urlsToWatch` diagnostic check

[garrytrinder]

Currently we show an error when the urlsToWatch array is empty, however

• urlsToWatch is optional
• urlsToWatch can be defined on a plugin instance

We should update the check so that an error is shown if plugins don't have urlsToWatch set and there is no global array set, or the global array is empty.

[garrytrinder]

Rather than continue with this, we should remove the check as urlsToWatch is no longer required in the config file as they can be passed as arguments, instead we should remove this check.