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)

src/main.rs

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