Revert "Forbid clippy warnings"

This reverts commit c0942e12a3.

1 | #![cfg_attr(feature = "cargo-clippy", deny(clippy::all))]
  |                                            ^^^^^^^^^^^
error[E0658]: scoped lint `clippy::all` is experimental (see issue #44690)

ci/script.sh

@@ -5,7 +5,7 @@ set -ex
 # TODO This is the "test phase", tweak it as you see fit
 main() {
     cargo fmt --all -- --check
-    cargo clippy --all-targets --all-features
+    cargo clippy --all-targets --all-features -- -D warnings
     cross check --target $TARGET
     cross check --target $TARGET --release
 

src/main.rs

@@ -1,5 +1,3 @@
-#![cfg_attr(feature = "cargo-clippy", deny(clippy::all))]
-
 extern crate directories;
 extern crate failure;
 extern crate which;