net: mvpp2: remove unused register definition MVPP2_TXQ_THRESH_REG

This register is no longer used since commit edc660fa ("net: mvpp2:
replace TX coalescing interrupts with hrtimer").

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>