arm64: extable: remove unused ex_handler_t definition

The ex_handler_t type was introduced in commit d6e2cc56

 ("arm64:
extable: add `type` and `data` fields"), but has never been used, and
is unnecessary. Remove it.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20211119124608.3f03380b@xhacker
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>