Fix minor clippy lints in grandpa (#5988)

* grandpa: fix clippy lints about identity conversions

* grandpa: fix clippy lints about unwrap_or_default

* grandpa: fix clippy lints about explicit return

* grandpa: fix clippy lints about unnecessary intermediary

* grandpa: fix clippy lints about to_string

* grandpa: fix clippy lints about unused imports

* grandpa: fix clippy lints about increments

* grandpa: fix clippy lints about unnecessary matches

* grandpa: fix clippy lints about struct arguments

* Fix clippy::redundant_clone

* Fix clippy::clone_on_copy

* Fix clippy::or_fun_call

* Fix clippy::identity_conversion