Remove SockLevel

It's been unused since PR #133

CHANGELOG.md

@@ -19,6 +19,9 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Removed
 
+- Removed `SockLevel`, which hasn't been used for a few years
+  (#[1362](https://github.com/nix-rust/nix/pull/1362))
+
 ## [0.19.1] - 28 November 2020
 ### Fixed
 - Fixed bugs in `recvmmsg`.

src/sys/socket/mod.rs

@@ -1576,24 +1576,6 @@ pub fn send(fd: RawFd, buf: &[u8], flags: MsgFlags) -> Result<usize> {
  *
  */
 
-/// The protocol level at which to get / set socket options. Used as an
-/// argument to `getsockopt` and `setsockopt`.
-///
-/// [Further reading](http://pubs.opengroup.org/onlinepubs/9699919799/functions/setsockopt.html)
-#[repr(i32)]
-#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
-pub enum SockLevel {
-    Socket = libc::SOL_SOCKET,
-    Tcp = libc::IPPROTO_TCP,
-    Ip = libc::IPPROTO_IP,
-    Ipv6 = libc::IPPROTO_IPV6,
-    Udp = libc::IPPROTO_UDP,
-    #[cfg(any(target_os = "android", target_os = "linux"))]
-    Netlink = libc::SOL_NETLINK,
-    #[cfg(any(target_os = "android", target_os = "linux"))]
-    Alg = libc::SOL_ALG,
-}
-
 /// Represents a socket option that can be accessed or set. Used as an argument
 /// to `getsockopt`
 pub trait GetSockOpt : Copy {