drivers: eth_mcux: remove interrupt clearing code

Remove code that clears spurious interrupt flags, as this step is
not required.

Performance change (as tested with iperf on RT1050)
TCP RX: 50.8Mbps->50.6Mbps
TCP TX: 46.2Mbps->49.5Mbps

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>