arm64: remove arm64ksyms.c

Now that arm64ksyms.c has been reduced to a stub, let's remove it
entirely. New exports should be associated with their function
definition.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>