std::ffi: Remove unused link

Author: tbu-

library/std/src/ffi/mod.rs

@@ -144,7 +144,6 @@
 //!
 //! [Unicode scalar value]: https://www.unicode.org/glossary/#unicode_scalar_value
 //! [Unicode code point]: https://www.unicode.org/glossary/#code_point
-//! [`env::set_var()`]: crate::env::set_var "env::set_var"
 //! [`env::var_os()`]: crate::env::var_os "env::var_os"
 //! [unix.OsStringExt]: crate::os::unix::ffi::OsStringExt "os::unix::ffi::OsStringExt"
 //! [`from_vec`]: crate::os::unix::ffi::OsStringExt::from_vec "os::unix::ffi::OsStringExt::from_vec"