alpha: locks: remove unused arch_*_relax operations

The arch_{spin,read,write}_relax macros are not used anywhere in the
kernel and are typically just aliases for cpu_relax().

This patch removes the unused definitions for Alpha.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>