[CI] Remove the redundant step "Build and compile assets"

[rosier]

The step "Install dependencies" with comoser install also runs the commands importmap:install and sass:build.

Also I think the tests should be able to run without asset-map:compile.

[javiereguiluz]

I like this, because less config is less maintenance for us. Thanks Jan!

My guess is that tests will be (slightly) faster after running asset-map:compile ... but the difference will be really small in CI tests, so let's merge this.