xtask: replace "lint" command by a simply cargo alias

This strips the run_clippy implementation out of xtask and replaces it by
a simple "cargo lint" alias which runs clippy with the corresponding flags.

Unfortunately  I could not name the alias "clippy" because that would lead to infinite recursion.

xtask/src/main.rs

@@ -40,7 +40,6 @@ fn main() -> Result<()> {
             return Ok(());
         }
         flags::XtaskCmd::Install(cmd) => cmd.run(),
-        flags::XtaskCmd::Lint(_) => run_clippy(),
         flags::XtaskCmd::FuzzTests(_) => run_fuzzer(),
         flags::XtaskCmd::PreCache(cmd) => cmd.run(),
         flags::XtaskCmd::Release(cmd) => cmd.run(),
@@ -95,25 +94,6 @@ fn ensure_rustfmt() -> Result<()> {
     Ok(())
 }
 
-fn run_clippy() -> Result<()> {
-    if cmd!("cargo clippy --version").read().is_err() {
-        bail!(
-            "Failed run cargo clippy. \
-            Please run `rustup component add clippy` to install it.",
-        )
-    }
-
-    let allowed_lints = "
-        -A clippy::collapsible_if
-        -A clippy::needless_pass_by_value
-        -A clippy::nonminimal_bool
-        -A clippy::redundant_pattern_matching
-    "
-    .split_ascii_whitespace();
-    cmd!("cargo clippy --all-features --all-targets -- {allowed_lints...}").run()?;
-    Ok(())
-}
-
 fn run_fuzzer() -> Result<()> {
     let _d = pushd("./crates/syntax")?;
     let _e = pushenv("RUSTUP_TOOLCHAIN", "nightly");