Fix nightly clippy errors

[thanethomson]

In working towards #978, some new clippy errors are popping up in CI with the latest Rust nightly. See https://github.com/informalsystems/tendermint-rs/pull/978/checks?check_run_id=3635238166 for an example.

This PR attempts to address these errors. If acceptable, I'll merge to master and then pull these changes into #978.

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Added entry in .changelog/

[thanethomson]

This error seems to be addressed in #978.

Also, I just confirmed that this change applied to #978 fixes the current documentation build failure in #978. Without this change, we get:

> RUSTDOCFLAGS="--cfg docsrs" cargo +nightly doc --all-features

...

error: field is never read: `db`
  --> light-client/src/store/sled.rs:26:5
   |
26 |     db: sled::Db,
   |     ^^^^^^^^^^^^
   |
note: the lint level is defined here
  --> light-client/src/lib.rs:3:5
   |
3  |     warnings,
   |     ^^^^^^^^
   = note: `#[deny(dead_code)]` implied by `#[deny(warnings)]`

error: field is never read: `chain_id`
  --> light-client/src/tests.rs:87:5
   |
87 |     chain_id: String,
   |     ^^^^^^^^^^^^^^^^

error: could not compile `tendermint-light-client` due to 2 previous errors

[xla]

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