mmap engine: remove unused variable mmap_map_mask

mmap engine: remove unused variable mmap_map_mask

The global variable 'mmap_map_mask' was introduced in this commit:

commit ac8931124126 ("mmap engine: allow large files on 32-bit archs")

and was used in fio_mmapio_prep() to calculate f->mmap_off.  This usage was
removed in this commit:

commit ff455a04f4e7 ("mmap engine: fix for large files on 32-bit archs")

but the definition and setup code for 'mmap_map_mask' were left intact.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>