docs: Remove old texinfo sources

docs: Remove old texinfo sources

We can now delete the old .texi files, which we have been keeping in
the tree as a parallel set of documentation to the new rST sources.
The only remaining use of Texinfo is the autogenerated manuals
and HTML documents created from the QAPI JSON doc comments.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Kashyap Chamarthy <kchamart@redhat.com>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200228153619.9906-33-peter.maydell@linaro.org