Revert "Add `FlxGroup::revive()`"

The following commit has been undone:
https://github.com/FlixelCommunity/flixel/pull/110

And a tiny bit of documentation has been added describing the changes.

Discussion:
 - https://github.com/FlixelCommunity/flixel/pull/185
 -
https://github.com/FlixelCommunity/flixel/issues/30#issuecomment-26753623
Merge pull request #187 from IQAndreas/revert-flxgroup-revive

Revert "Add `FlxGroup::revive()`"