arm64: lse: Remove unused 'alt_lse' assembly macro

The 'alt_lse' assembly macro has been unused since 7c8fc35d
("locking/atomics/arm64: Replace our atomic/lock bitop implementations
with asm-generic").

Remove it.

Reviewed-by: Andrew Murray <andrew.murray@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>