[SCSI] megaraid_sas: remove poll_mode_io code

This code has never worked correctly, doesn't disable interrupts when
set as a module parameter, doesn't disable interrupts when set after
driver load time in sysfs node, etc.

Signed-off-by: Adam Radford <aradford@gmail.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>