Make the sass options task-specific rather than target-specific

Same end-result, but makes it more consistent with other tasks.
Delete the style-old.css

This was a cut down of the original style.css, pre-Sass.
Remove styledocco

Styledocco is for documenting style sheets. It doesn't make sense to pollute
the end style.css with documentation, in a project that uses Sass. Any
documentation can be included in the .scss files, and ths removed from the
compiled .css file(s).

With Styledocco gone, the replace task can be simplified. Replacing /*! with /*
in the style.dev.css just to make Styledocco parse correctly, is no longer
needed.

(The replacement for ensuring a clear linespace after each ruleset is also
removed, as this is now handled by CSSComb.)

With the replacements simplified, the timing of the copy can also be simplified.
The copy is now done on the single style.dev.css file, and the minification is
now done when this file has been moved to the theme directory. The remaining
replacement (adding a clear linespace after the banner) is now done after that.