CornerPointChopper: Tidy code to output GRDECL format of subsample

This change-set tidies the code used to implement method `writeGrdecl()`.  Specifically

 * Use method `outputField()` to output the `COORD` and `ZCORN` arrays, thus eliminating repetition
 * Simplify the implementation of `outputField()` to use straight-line code rather than a nested `for()` loop

While here, also remove a long-disabled code section for overall file readability.