Remove various modules which have been completely deprecated by the following tickets:

• Base Polyhedron on PPL (by default) #11634 (Sage-5.0)
• Move preparsing, loading and attaching to src/sage/repl #17396 (Sage-6.5)
• Deprecate src/sage/misc/db.py #17653 (Sage-6.6)
• Remove deprecated group_algebra.py #17779 (Sage-6.6)
• get rid of MatrixSpace_ZZ_2x2 #17824 (Sage-6.6)

Component: misc

Author: Jeroen Demeyer

Branch: 474a78d

Reviewer: Vincent Delecroix

Issue created by migration from https://trac.sagemath.org/ticket/20344