Removing the last uses of is_GapElement, deprecating it

Depends on #34770

CC: @mkoeppe

Component: interfaces

Author: Dima Pasechnik

Branch/Commit: e99db3a

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/34823