arm64: atomics: Remove atomic_ll_sc compilation unit

We no longer fall back to out-of-line atomics on systems with
CONFIG_ARM64_LSE_ATOMICS where ARM64_HAS_LSE_ATOMICS is not set.

Remove the unused compilation unit which provided these symbols.

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