Remove to options value

When passing `to` it truncates the output source map file path to just the path base name. By removing this value the paths are now correct.

The implementation of this logic is found [here](https://github.com/postcss/postcss/blob/master/lib/map-generator.es6#L179-L195)

Author: camshaft

lib/index.js

@@ -103,7 +103,6 @@ module.exports = function loader (css, map, meta) {
 
     let plugins = config.plugins || []
     let options = Object.assign({
-      to: file,
       from: file,
       map: sourceMap
         ? sourceMap === 'inline'