Remove user-select rule; there is no standard 'user-select' property

'user-select' *was* included in an early working draft:

<http://www.w3.org/TR/1999/WD-css3-userint-19990916#user-select>

but was then superseded:

<http://www.w3.org/TR/css3-userint>

by:

<http://www.w3.org/TR/css3-ui/>

which dropped all references to a 'user-select' property.
There is no w3 standard 'user-select' property, but there are three
[differently implemented] vendor-prefixed implementations.

Reference: <https://developer.mozilla.org/docs/CSS/user-select>