pctrie: drop meaningless neighbor check

In PCTRIE_INSERT_LOOKUP_{G,L}E, there is a test - if two pointers are
equal, replace one with a new value. The pointers can never be equal;
one points to a struct pctrie_node and the other is the (void*) cast
of a pointer to a field within a struct pctrie_node. So the tests and
assignments can be removed with no effect.

Reviewed by: bnovkov
Differential Revision: https://reviews.freebsd.org/D47277