Replace unnecessary `&mut self` with `&self` in `BlockImport::import_block()` (#5339)
There was no need for it to be `&mut self` since block import can happen concurrently for different blocks and in many cases it was `&mut Arc<dyn BlockImport>` anyway :man_shrugging: Similar in nature to https://github.com/paritytech/polkadot-sdk/pull/4844