s390/irq: remove HAVE_IRQ_EXIT_ON_IRQ_STACK

s390/irq: remove HAVE_IRQ_EXIT_ON_IRQ_STACK

[ Upstream commit 0aa4ff7688632a86bdb133fa106f2ccd514b91a7 ]

This is no longer true since we switched to generic entry. The code
switches to the IRQ stack before calling do_IRQ, but switches back
to the kernel stack before calling irq_exit().

Fixes: 56e62a737028 ("s390: convert to generic entry")
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>