Update _grid.scss

Clean up code and prevent possible duplicated output:
* the mixin `grid-column` produced duplicated output if called with `$columns:x, $last-column:false, $collapse:true`
* the implicit option of `$collapse:null` to skip the padding properties remains

Tests done: successfully compiled to grid HTML classes identical to before the change. (Duplicated output did not occur in the HTML classes generation because the given parameter combination was never used.)