Remove pre-2005 vendor-prefixed -khtml- CSS property

Summary:
Remove -khtml-opacity: 0.5; from CSS.
Per https://developer.apple.com/library/archive/documentation/AppleApplications/Reference/SafariCSSRef/Articles/StandardCSSProperties.html, standard opacity has been supported since Safari 2.0 released in April 2005. If you still run Safari 1.x after 20 years you have bigger problems than this line of CSS in Phorge.
This is the only -khtml- prefixed property in the codebase.

Test Plan: None.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25914