rustdoc: remove legacy user-select CSS

rustdoc: remove legacy user-select CSS

According to [caniuse], the only supported browser that requires the vendor prefix,
as defined in [RFC 1985], is Safari.

* The last version of Chrome that required a vendor prefix was version 53.
  The current version is 108.
* Firefox 68 is the last version that required a vendor prefix. The
  [current Firefox ESR] is version 102.
* The current version of Safari for Mac and iOS still requires a prefix.
* The last version of Edge that required a vendor frefix was 18. The current
  version of Edge is 108.
* UCAndroid support is unknown, but if it still requires a vendor prefix,
  it's more likely to be `-webkit-` than `-moz-` or `-ms-`, since they
  would want to emulate iOS for compatibility.

[caniuse]: https://caniuse.com/?search=user-select
[RFC 1985]: https://rust-lang.github.io/rfcs/1985-tiered-browser-support.html
[current Firefox ESR]: https://wiki.mozilla.org/Releases