rustdoc: clean up `.out-of-band`/`.in-band` CSS
| | method | impl
|--------|--------|------
| before |  | 
| after |  | 
* Remove the `float: right` fallback from the main header, which hasn't been needed since IE11 support was dropped.
* Remove `in-band` from low-level headers, which hasn't been needed since `.rightside` switched to `float: right` in
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| .. | ||
| assembly | ||
| auxiliary | ||
| codegen | ||
| codegen-units | ||
| debuginfo | ||
| incremental | ||
| mir-opt | ||
| pretty | ||
| run-make | ||
| run-make-fulldeps | ||
| run-pass-valgrind | ||
| rustdoc | ||
| rustdoc-gui | ||
| rustdoc-js | ||
| rustdoc-js-std | ||
| rustdoc-json | ||
| rustdoc-ui | ||
| ui | ||
| ui-fulldeps | ||
| COMPILER_TESTS.md | ||