Removed obsolete sourcemapping option

At the development parsing of Sass files, there is currently a sourcemap option given at `grunt/options/sass.js`.

    Running "sass:dev" (sass) task
    DEPRECATION WARNING: Passing --sourcemap without a value is deprecated.
    Sourcemaps are now generated by default, so this flag has no effect.

I'd be happy to hear if there are any reasons why it is still given there. Anyway, I made a pull request about it.