Refactor: cib: remove dead code

cib_process_delete_absolute() contained the line:

CRM_CHECK(strcasecmp(CIB_OP_DELETE, op) == 0, return -EINVAL);

However it was only registered for CIB_OP_DELETE_ALT, so it would never do
anything but return -EINVAL. It is never used internally, and obviously it
hasn't been used externally either, since it hasn't worked since the function
was created in 4d68c7211 in 2008, and no one's complained.

So, just delete its code and return -EINVAL. At a backward compatibility break,
we can drop it altogether.