Fix "explicitly defaulted but implicitly deleted" warning

`PhysicalCore`'s move assignment operator was declared as `= default`,
but was implicitly deleted because `PhysicalCore` has fields
of reference type.  Switch to explicitly deleting it to avoid a Clang
warning.

The move *constructor* is still defaulted, and is required to exist due
to the use of `std::vector<PhysicalCore>`.