Rollup merge of rust-lang#102380 - notriddle:notriddle/rustdoc-source-sidebar, r=camelid

rustdoc: remove redundant mobile `.source > .sidebar` CSS

When the source sidebar and standard sidebar had most of their code merged in 07e3f99, the properties `z-index: 11`, `margin: 0`, and `position: fixed` were already being set on the `.sidebar` class, so no need to repeat them.

https://github.com/rust-lang/rust/blob/57ee5cf5a93923dae9c98bffb11545fc3a31368d/src/librustdoc/html/static/css/rustdoc.css#L1742-L1754

Author: JohnTitor