intrng: Remove irq_root_ipicount and corresponding intr_pic_claim_root arg

The static irq_root_ipicount variable is only ever written to (with the
value passed to irq_root_ipicount), never read. Moreover, the bcm2836
driver, as used by the Raspberry Pi 2B and 3A/B (but not 4, which uses a
GIC-400, though does have the legacy interrupt controller present too)
passes 0 as ipicount, despite implementing IPIs. It's thus inaccurate
and serves no purpose, so should be removed.

Reviewed by: mmel, imp, mhorne
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D35897