Some clippy errors after updating rust toolchain

[viirya]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

There are some clippy errors after updating rust toolchain, e.g.:

error: parameter is only used in recursion
   --> arrow-schema/src/field.rs:208:24
    |
208 |     fn _fields<'a>(&'a self, dt: &'a DataType) -> Vec<&Field> {
    |                        ^^^^
    |
    = note: `-D clippy::only-used-in-recursion` implied by `-D warnings`
note: parameter used here
   --> arrow-schema/src/field.rs:217:53
    |
217 |             DataType::Dictionary(_, value_field) => self._fields(value_field.as_ref()),
    |                                                     ^^^^
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#only_used_in_recursion

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