IRQ_TYPE_SLOW is no longer used by the Atari platform interrupt code
since commit 73408565 ("[PATCH] m68k: convert atari irq code")
in v2.6.18-rc1, so drop it.

Note that its value has been reused for a different purpose
(IRQ_TYPE_NONE) since commit 6a6de9ef ("[PATCH] genirq: core")
in v2.6.18-rc1.
Signed-off-by: NGeert Uytterhoeven <geert@linux-m68k.org>