style: Remove explicit lifetimes

[caspermeijn]

This solves clippy warnings like this:

error: the following explicit lifetimes could be elided: 'a
   --> graphql_client_codegen/src/codegen/selection.rs:394:6
    |
394 | impl<'a> ExpandedField<'a> {
    |      ^^                ^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
    = note: `-D clippy::needless-lifetimes` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::needless_lifetimes)]`
help: elide the lifetimes
    |
394 - impl<'a> ExpandedField<'a> {
394 + impl ExpandedField<'_> {
    |