Grunt: tweak `build-dev` to stop running both varients of `cssjanus`.

`cssjanus:dynamic` is only intended to be used by the `grunt watch` feature. See #5160

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8387 cdf35c40-ae34-48e0-9cc9-0c9da1808c22