IB/qib: Remove DMA mapping code

The qib DMA mapping code is no longer built since commit eb636ac0
("IB/qib: Remove dma.c and use rdmavt version of dma functions"). Hence
remove it.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Mike Marciniszyn <mike.marciniszyn@intel.com>
Cc: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>