Rollup merge of #102491 - notriddle:notriddle/sidebar-opacity, r=GuillaumeGomez

Rollup merge of #102491 - notriddle:notriddle/sidebar-opacity, r=GuillaumeGomez

rustdoc: remove no-op source sidebar `opacity`

These rules were added in dc2c9723343c985740be09919236a6e96c4e4433 to work with CSS transitions. They're otherwise redundant, since the `visibility` property already hides everything.

https://github.com/rust-lang/rust/blob/dc2c9723343c985740be09919236a6e96c4e4433/src/librustdoc/html/static/css/rustdoc.css#L350-L354

The transition was remove with 237d62588ddb4b7a93f3f5c61ea9253eba30ed5d, but the now-redundant `opacity` property was not.