irqdesc: Delete irq_desc_get_msi_desc()

Function irq_desc_get_msi_desc() is not referenced in the kernel (and does
not seem to have been referenced since e39758e0

, 3 years ago), so
delete it.

Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: <marc.zyngier@arm.com>
Cc: <will.deacon@arm.com>
Cc: <kstewart@linuxfoundation.org>
Cc: <julien.thierry@arm.com>
Cc: <andrew@lunn.ch>
Cc: <trivial@kernel.org>
Link: https://lkml.kernel.org/r/1529667333-92959-1-git-send-email-john.garry@huawei.com