guix: document when certain guix patches can be dropped, remove redundant glibc patches

Summary:

guix: document when certain guix patches can be dropped

Note that we don't support powerpc, and the glibc-2.28-fcommon patch mentions that it specifically fixes an issue for this platform, but the patch is applied by Core to all builds, so it is best to keep it.

guix: remove redundant glibc patches

These should only be relevant for a glibc that is built as part of a
Guix system, and should not be required for a glibc that is just being
built to compile our binaries against. A x86_64 linux bitcoind produced
with Guix using master vs this change has no difference. i.e:

This is a backport of core#27668 and core#27670
Depends on D15330

Test Plan: contrib/guix/guix-build

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15331