Remove unnecessary returns

These were flagged by `cargo clippy`:

    warning: unneeded `return` statement

src/socket/set.rs

@@ -75,7 +75,7 @@ impl<'a, 'b: 'a, 'c: 'a + 'b> Set<'a, 'b, 'c> {
             ManagedSlice::Owned(ref mut sockets) => {
                 sockets.push(None);
                 let index = sockets.len() - 1;
-                return put(index, &mut sockets[index], socket)
+                put(index, &mut sockets[index], socket)
             }
         }
     }

src/wire/ndiscoption.rs

@@ -596,7 +596,7 @@ impl<T: AsRef<[u8]>> PrettyPrint for NdiscOption<T> {
             Err(err) => return write!(f, "{}({})", indent, err),
             Ok(ndisc) => {
                 match Repr::parse(&ndisc) {
-                    Err(_) => return Ok(()),
+                    Err(_) => Ok(()),
                     Ok(repr) => {
                         write!(f, "{}{}", indent, repr)
                     }