fix(ci): remove all mentions of webpack 3
fix(ci): remove all mentions of webpack 3

Since we do not support webpack 3 anymore,
let us remove entirely the tests for it.

vue-styleguidist/vue-styleguidist,vue-styleguidist/vue-styleguidist,vue-styleguidist/vue-styleguidist,vue-styleguidist/vue-styleguidist