Fix SVG optimization so it doesn't remove symbols for external <use> tags

[lhorie]

Rationale: we want users to be able to take an SVG file such as this:

<svg xmlns="http://www.w3.org/2000/svg">
  <symbol id="mything" viewBox="0 0 24 24">
    <path d="somepath" fill="var(--thing-color)"/>
  </symbol>
</svg>

And use it like this:

const mything = `${assetUrl('../static/mything.svg')}#mything`;

// JSX
<svg width="24" height="24" style={{'--thing-color': '#00f'}}>
  <use xlinkHref={mything} />
</svg>

This PR disables optimizations that prevents this use case from working.

[KevinGrandon]

Nice!