GNUInstallDirs: Revert "Apply Debian multiarch LIBDIR to more prefixes"

Revert commit dd9584b3 (GNUInstallDirs: Apply Debian multiarch LIBDIR
to more prefixes, 2021-11-19, v3.23.0-rc1~323^2).  There are separate
problems with activating multiarch `LIBDIR` for each prefix it added:

* Prefix `/` is often used to stage an installation with `DESTDIR`
  for inclusion in a tarball package or similar.

* Prefix `/usr/local` is the default `CMAKE_INSTALL_PREFIX`, causing
  the multiarch `LIBDIR` to be cached after the first configuration,
  even if the prefix changes later.

Revert the change for now, except for the documentation update.
Further discussion will be needed to select a way to enable
multiarch `LIBDIR` for `/` and `/usr/local`.

Fixes: #23365
Issue: #19698
42 jobs for !7127 with GNUInstallDirs-revert-multiarch-prefixes in 47 minutes and 48 seconds (queued for 4 seconds)
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