Rust's linting API is incredibly unstable, resulting in unnecessary
breakage to `rocket_codegen`. Rocket's lints are also not as
conservative as would be desired, resulting in spurious warnings. For
these reasons, this commit removes linting from `rocket_codegen`.
These lints will likely be reintroduced as part of a 'rocket_lints'
crate. Factoring the lints out to a separate crate means that lint
breakage can be dealt with by uncommenting the dependency instead of
waiting for a new release or backtracking nightlies. In the same vein,
it will likely improve stability of the 'rocket_codegen' crate.
Sergio Benitez