chore(cpus): remove redundant asserts

get_cpu_ops_ptr asserts that it didn't get 0 for a cpu_ops pointer. Its
callers don't need to do the same.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I547ac592949f74e153ef161015326f64aead2f28