Bitrig support has been removed

Bitrig support has been removed (by me) in rust-lang/rust#60775

Author: hellow554

src/conditional-compilation.md

@@ -123,7 +123,6 @@ Example values:
 * `"android"`
 * `"freebsd"`
 * `"dragonfly"`
-* `"bitrig"`
 * `"openbsd"`
 * `"netbsd"`