arm64: asm-offsets: remove unused offsets

There are a number of offsets defined in asm-offsets.c which no longer
have any users. Let's clean this up by removing them.

All the remaining offsets are in use.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Andrew Murray <andrew.murray@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>