Don't build ol.css in check target

Currently the `check` build target builds `build/ol.css`.
`build/ol.css` is a side effect of building `build/ol.js`.
`build/ol.js` takes time and only compiles code that is reachable from exported functions.

`build/ol-all.js` attempts to compile all code, and therefore has better coverage that `build/ol.js`.

This PR changes the `check` target so that it builds `build/ol-all.js` only.