Removed misleading comments and legacy requirement calls

The instructions in the init() function seem to be at odds with the current documentation which suggests that it is preferable to include requirements in PHP code rather than in templates.

Also the requirement calls here are supposedly included so that older themes still work - but how old are we talking? I would assume that these older themes would not be compatible with SS3.1, and the bundled Simple theme does not need these requirement calls in the init() function, making them redundant.

I am guessing that the Installer is primarily used by people just getting started with SilverStripe, so ideally this bundled example Page.php should provide a clear pathway to best practice SS development, rather than supporting out-dated code.