fix memory leak via NeighListKokkos::clean_copy()

There were several clean_copy() calls in pair
styles *outside device code*.
They seem to have been left over from an abandoned
effort to copy the Kokkos neighbor list as
a member of the pair style, instead of copying
out the individual views needed.
These leftover clean_copy() calls were setting
pointers to NULL that had not been freed,
leading to large memory leaks.
I've removed the clean_copy() function entirely,
and replaced it with the copymode flag system used
in many other Kokkos objects.
The copymode flag is only set to one in
functors that hold copies of the neighbor list.