frv: Remove stale irq_chip.end

irq_chip.end got obsolete with the removal of __do_IRQ().

irq-mb93093.c even lacks an implementation, but nobody noticed that
it's broken since commit 88d6e1 in 2006.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: David Howells <dhowells@redhat.com>