docs: Remove unused weirdly-named cross-reference targets

docs: Remove unused weirdly-named cross-reference targets

In the doc sources, we have a few cross-reference targets with odd
names "pcsys_005fxyz".  These are the legacy of the semi-automated
conversion of the old info docs to rST (the '005f' is because ASCII
0x5f is '_' and the old info link names had underscores in them).

Remove the targets which nothing links to, and rename the two targets
which are used to something a bit more descriptive.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20230421163642.1151904-1-peter.maydell@linaro.org
Reviewed-by: Markus Armbruster <armbru@redhat.com>