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

The SWC compiles without any errors, but I have not tested the changes in a project.