Remove unused imports/ doc comments. (#2276)

* Remove unused imports/ doc comments

* Better doc for node template.

substrate/core/client/db/src/lib.rs

@@ -1122,8 +1122,6 @@ impl<Block> client::backend::Backend<Block, Blake2Hasher> for Backend<Block> whe
 	}
 
 	fn revert(&self, n: NumberFor<Block>) -> Result<NumberFor<Block>, client::error::Error> {
-		use client::blockchain::HeaderBackend;
-
 		let mut best = self.blockchain.info()?.best_number;
 		let finalized = self.blockchain.info()?.finalized_number;
 		let revertible = best - finalized;

substrate/core/client/src/client.rs

@@ -675,8 +675,6 @@ impl<B, E, Block, RA> Client<B, E, Block, RA> where
 	) -> error::Result<ImportResult> where
 		E: CallExecutor<Block, Blake2Hasher> + Send + Sync + Clone,
 	{
-		use runtime_primitives::traits::Digest;
-
 		let ImportBlock {
 			origin,
 			header,