Remove unused prevent-user-select mixin

[sandbergja]

What maintenance needs to be done?

Remove prevent-user-select from the system.scss and mixins.scss. Release a major version of Lux.

Level of urgency

• High
• Moderate
• Low

Why is this maintenance needed?

Cleanup unused scss code. This mixin also references non-standard CSS properties that are no longer needed.

This is a breaking change, so we should release a major release after removing this.

Acceptance criteria

• prevent-user-select mixin is removed from lux
• we release a major release