firmware: arm_sdei: remove unused interfaces

The export symbols to register/unregister and enable/disable events
aren't used upstream, remove them.

[ dropped the parts of Christoph's patch that made the API static too ]

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Morse <james.morse@arm.com>
Link: https://lore.kernel.org/linux-arm-kernel/20200504164224.2842960-1-hch@lst.de/
Link: https://lore.kernel.org/r/20200519182108.13693-3-james.morse@arm.com


Signed-off-by: Will Deacon <will@kernel.org>