Removed unnecessary cms require when building framework

If the build is run off `silverstripe/framework`, it shouldn't include the CMS module - both because it might introduce side effects, and because it'd mean we don't test framework on its own which is bad.

If the build is run for a module requiring both cms and framework, the conditional below the removed one will take effect (starting with comment `Override module dependencies...`).

If the build is run for a 2.x codebase, the changed conditional above the removed code will take effect.

See https://github.com/silverstripe/silverstripe-framework/pull/3132#issuecomment-51547149 for context.