Remove legacy admin/cms route

[GuySartorelli]

I've just come across this routing redirection from the legacy admin/cms to admin/pages:

silverstripe-cms/_config/routes.yml

Lines 8 to 13 in df82162

	---
	Name: legacycmsroutes
	---
	SilverStripe\Control\Director:
	rules:
	'admin/cms': '->admin/pages'

I strongly doubt anyone needs this redirection anymore. Let's get rid of it.

Acceptance criteria

• The rerouting from admin/cms to admin/pages is removed