Remove feature not required by Ipv6Addr::to_cononical doctest

The feature does not seem to be required by this doctest.

Author: eopb

library/core/src/net/ip_addr.rs

@@ -1860,7 +1860,6 @@ impl Ipv6Addr {
     /// # Examples
     ///
     /// ```
-    /// #![feature(ip)]
     /// use std::net::Ipv6Addr;
     ///
     /// assert_eq!(Ipv6Addr::new(0, 0, 0, 0, 0, 0xffff, 0x7f00, 0x1).is_loopback(), false);