MAINTAINERS: Fix bad / stale F: in "virtio-user-rng"

tools/vhost-user-rng/* never existed.  Drop the bad line.

We neglected to update MAINTAINERS when we moved
docs/system/devices/vhost-user-rng.rst to docs/system/devices/virtio/.
We later deleted the file.  Drop the stale line.

Fixes: e3e01466bf (MAINTAINERS: Add maintainer for vhost-user RNG implementation, 2021-06-14)
Fixes: 487ce6ce99 (docs/system: split VirtIO devices from the rest, 2025-10-16)
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20260603064032.3733394-23-armbru@redhat.com>