mmap: remove unused 'use'
mmap: remove unused 'use'

usize isn't used in this test any more. Remove compiler warning.

Change-Id: I9493e22f563f2fc15532564ea57d70de593c4421
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/902778
Reviewed-by: Sonny Rao <d867fc987a366be56684d1145622524a3ba1efbf@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm