Fix <SimpleListLoading> link style is not implemented error.

[WiXSL]

You get an error using <SimpleListLoading> component :

Material-UI: the key `link` provided to the classes prop is not implemented in undefined.
You can only override one of the following: primary,tertiary. 
    in SimpleListLoading (created by SimpleList)
    in SimpleList (at licencia.js:61)
    in LicenciaList (created by WithPermissions)
    in WithPermissions (created by Router.Consumer)
    in Router.Consumer (created by Route)
    in Route (created by ResourceRoutes)
    in ResourceRoutes (created by Resource)
    in Resource (at App.js:47)
    in App (at src/​index.js:7)`
`link` style is defined in `<SimpleList>`

[fzaninotto]

The styles for LinkOrNot cannot be overridden by the user. So instead of having one useStyles hook for two components, we'd better have two styles hooks, one for each component.

[WiXSL]

Well, LinkOrNot is never exported anyways, should I change it to another file and export it as well?
Maybe in another PR?

[fzaninotto]

No, it's fine like that, thanks.