IB/mthca: Remove MSI support as scheduled

Remove MSI support from the mthca driver, as scheduled.  There is no
reason to use MSI instead of MSI-X, since MSI-X performs better.  No
one has spoken up since MSI support was deprecated in commit f6be6fbe
("IB/mthca: Schedule MSI support for removal"), so apparently the MSI
support is unused.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Roland Dreier <rolandd@cisco.com>