Resolve redundant_guards clippy lint

    warning: redundant guard
      --> build/rustc.rs:60:26
       |
    60 |         Some(channel) if channel == "dev" => Dev,
       |                          ^^^^^^^^^^^^^^^^
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_guards
       = note: `-W clippy::redundant-guards` implied by `-W clippy::all`
    help: try
       |
    60 -         Some(channel) if channel == "dev" => Dev,
    60 +         Some("dev") => Dev,
       |

    warning: redundant guard
      --> build/rustc.rs:62:26
       |
    62 |         Some(channel) if channel == "nightly" => match words.next() {
       |                          ^^^^^^^^^^^^^^^^^^^^
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_guards
    help: try
       |
    62 -         Some(channel) if channel == "nightly" => match words.next() {
    62 +         Some("nightly") => match words.next() {
       |

build/rustc.rs

@@ -57,9 +57,9 @@ fn parse_words(words: &mut dyn Iterator<Item = &str>) -> Option<Version> {
 
     let channel = match channel {
         None => Stable,
-        Some(channel) if channel == "dev" => Dev,
+        Some("dev") => Dev,
         Some(channel) if channel.starts_with("beta") => Beta,
-        Some(channel) if channel == "nightly" => match words.next() {
+        Some("nightly") => match words.next() {
             Some(hash) if hash.starts_with('(') => match words.next() {
                 None if hash.ends_with(')') => Dev,
                 Some(date) if date.ends_with(')') => {