Seems we don't need cache

[jingsam]

In https://github.com/mapbox/sphericalmercator/blob/master/sphericalmercator.js#L36-L39

The cache object is not used anywhere.

[jingsam]

I see the cache. https://github.com/mapbox/sphericalmercator/blob/master/sphericalmercator.js#L62

But I doubt how necessary of the cache. Eliminate the cache would make code more concise and clean.