Theming: Remove IconOverlay section from default.creatortheme

IconOverlays are now defined in the Images section. Also,
default.creatortheme does not need to define these since they are
already defined in the cpp code as fallback.

See a18b067326bd58197dd70d4db9d9b79b01c2c8a9

Change-Id: Id2cf616068b11c5aa8e43ee695369063e6a4ee0e
Reviewed-by: Thorben Kroeger <thorbenkroeger@gmail.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>