xprtrdma: Remove Tavor MTU setting

Clean up.  Remove HCA-specific clutter in xprtrdma, which is
supposed to be device-independent.

Hal Rosenstock <hal@dev.mellanox.co.il> observes:
> Note that there is OpenSM option (enable_quirks) to return 1K MTU
> in SA PathRecord responses for Tavor so that can be used for this.
> The default setting for enable_quirks is FALSE so that would need
> changing.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>