tidy: cargo.rs: Clean up loop to use "for" instead of "while let"

Eliminates a clippy warning.

Also drop the unnecessary `.peekable()`.

src/tools/tidy/src/cargo.rs

@@ -65,8 +65,7 @@ fn verify(tomlfile: &Path, libfile: &Path, bad: &mut bool) {
         Some(i) => &toml[i+1..],
         None => return,
     };
-    let mut lines = deps.lines().peekable();
-    while let Some(line) = lines.next() {
+    for line in deps.lines() {
         if line.starts_with('[') {
             break
         }