Deleted Core Nav.

This has moved to Onward
Removed references to deleted Core Nav project
Merge branch 'master' of github.com:guardian/frontend into onwards-next-gen

Conflicts:
	core-navigation/app/controllers/MostPopularController.scala
Merge branch 'master' of github.com:guardian/frontend into onwards-next-gen
Merge branch 'onwards-next-gen' of github.com:guardian/frontend into onwards-next-gen
Merge branch 'master' of github.com:guardian/frontend into onwards-next-gen
Merge pull request #2923 from guardian/onwards-next-gen

Deleted Core Nav.