Remove grotty special-case code in coerce_to_target_type() that

Remove grotty special-case code in coerce_to_target_type() that
implemented casts to varchar and bpchar using a cast-to-text function.
This is a holdover from before we had pg_cast; it now makes more sense
to just list these casts in pg_cast.  While at it, add pg_cast entries
for the other direction (casts from varchar/bpchar) where feasible.