docs: remove qemu_logo.pdf

As far as I can tell nothing in the source tree uses this file. The
original comment mentions texi2pdf and I think that was all culled
when we switched to our rst documentation pipeline.

Fixes: 4ac2ee19 ("docs/interop: Convert qemu-qmp-ref to rST")
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220316161038.3033723-1-alex.bennee@linaro.org>