Removes remap_via_sub_cells(), after copying code into remapping_core_h()

This commit addresses a `\todo` added in #662, namely that
the functions remap_via_sub_cells() and remap_via_sub_cells_om4() could,
and should, be moved up into remapping_core_h(), as well as into the
obsolete remapping_core_w().

In that PR, the function remap_via_sub_cells() had been modularized into
three other functions, and the near-duplicatem remap_via_sub_cells_om4(),
called the two of the same functions as well as an out-of-date version of
the third function.

Since the reampping_core_h() function calls is already brief, calling one
function in addition to remap_via_sub_cells(), and since
remap_via_sub_cells() is only called from remapping_core_*(), we can remove
a level of call-tree depth depth by copying remap_via_sub_grid() into
reampping_core_h() and removing the unused function(s).

Author: adcroft