Unify user-select CSS directives

Summary:
Removes the -khtml, -moz and -ms prefix, since most Browsers are natively supporting the user select directive.
The -webkit prefix is still kept or added for Safari, wich does not support user-select.

Ref: see https://we.phorge.it/D25024#815 for context

Test Plan: Removing the CSS should change nothing in modern browsers.

Reviewers: O1 Blessed Committers, speck, valerio.bozzolan, Matthew

Reviewed By: O1 Blessed Committers, speck, valerio.bozzolan

Subscribers: Cigaryno, Matthew, speck, tobiaswiese, valerio.bozzolan

Differential Revision: https://we.phorge.it/D25025