cxgbe(4): Retire t4_intr_clear.

The firmware clears the interrupts already and it has a better idea of
exactly what to clear for which generation of the ASIC. There is no
need for the driver to get involved.

Sponsored by: Chelsio Communications

(cherry picked from commit 1c7f9c8b4673abf3723be09afed4443261e0d186)