build: Remove obsolete 'build' task from grunt-doc
build: Remove obsolete 'build' task from grunt-doc

This is already part of `npm run-script doc`, which is the intended
entry point. It was being run twice.

Change-Id: I8f3dd81052ce6ec6fa40542c24526ad1631ed745

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui