dataplane/xen-block: remove dead code

dataplane/xen-block: remove dead code

The if() statement is clearly bogus (dead code which should have been
cleaned up when grant mapping was removed).

Spotted by Coverity: CID 1398635

While in the neighbourhood, add a missing 'fall through' annotation.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20190215162533.19475-2-paul.durrant@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>