[PARISC] remove remnants of parisc-specific softirq code

Kyle,

This patch removes remnants of softirq support that we no longer need.
I suspect this was just overlooked when willy convert parisc to generic
IRQ support.

Tested on c3600 32-bit UP.

Signed-off-by: Grant Grundler <grundler@parisc-linux.org>
[and tested on a c8000 64-bit SMP --kyle]
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>