net: rtl8169: remove unneeded definition

The rtl8169_intr_mask variable isn't used anywhere in the code, so
just remove it to avoid a GCC 6.2 compiler warning.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>