Revert "Added a global CachedGraphics persistence variable."

This reverts commit d06e8021e645eda4fa9786776db9902169727d1b.
Add a global CachedGraphics persistence variable.

If specified, CachedGraphics will use the persist value supplied by the
constructor. Otherwise, it defaults to the value of
persistentCachedGraphics in FlxG.

This allows globally setting what the persist value should default to,
making it easier to keep desired assets persistent between state
changes.