Remove the file src/sage/algebras/group_algebra.py which was deprecated in #6670.

Rename src/sage/algebras/group_algebra_new.py to src/sage/algebras/group_algebra.py (with deprecation).

Component: algebra

Author: Jeroen Demeyer

Branch/Commit: dbb3d10

Reviewer: Vincent Delecroix

Issue created by migration from https://trac.sagemath.org/ticket/17779