Allow justifications on non-finalized blocks (paritytech#1211)

One assertion was unnecessary because of the check right above it,
second assertion resolves
paritytech#1159

---------

Co-authored-by: Bastian Köcher <[email protected]>

Author: nazar-pc

substrate/client/consensus/grandpa/src/import.rs

@@ -552,7 +552,6 @@ where
 							.into(),
 					))
 				}
-				assert!(block.justifications.is_some());
 				let mut authority_set = self.authority_set.inner_locked();
 				authority_set.authority_set_changes.insert(number);
 				crate::aux_schema::update_authority_set::<Block, _, _>(

substrate/client/service/src/client/client.rs

@@ -603,8 +603,6 @@ where
 			.block_gap
 			.map_or(false, |(start, _)| *import_headers.post().number() == start);
 
-		assert!(justifications.is_some() && finalized || justifications.is_none() || gap_block);
-
 		// the block is lower than our last finalized block so it must revert
 		// finality, refusing import.
 		if status == blockchain::BlockStatus::Unknown &&