With recent changes to `ArrayHandle`, the type for the associated array
portal is now the same across all devices. This means that almost all
exec objects no longer need to be specialized on the device types. Thus,
clean up the locator exec objects to no longer need to be templated on
device.
The new name reflects better what the underlying algorithm does. It also
helps prevent confusion about what types of data the locator is good
for. The old name suggested it only worked for structured grids, which
is not the case.