vfio/platform: Remove dead assignment in vfio_intp_interrupt()

vfio/platform: Remove dead assignment in vfio_intp_interrupt()

Clang static code analyzer show warning:
hw/vfio/platform.c:239:9: warning: Value stored to 'ret' is never read
        ret = event_notifier_test_and_clear(intp->interrupt);
        ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Message-Id: <20200827110311.164316-8-kuhn.chenqun@huawei.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>