IB/mthca: remove mthca_user.h

mthca_user.h is unused since commit 486f6095 ("IB/mthca: Move user vendor structures")
Remove it from tree.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>