Revert "Check for parent of first ready block being on chain (#1812)" (#1950)

This reverts https://github.com/paritytech/polkadot-sdk/pull/1812 until
we know why it causes syncing issues reported in
https://github.com/subspace/subspace/issues/2122.

substrate/client/network/sync/src/blocks.rs

@@ -212,31 +212,6 @@ impl<B: BlockT> BlockCollection<B> {
 		ready
 	}
 
-	/// Returns the block header of the first block that is ready for importing.
-	/// `from` is the maximum block number for the start of the range that we are interested in.
-	/// The function will return None if the first block ready is higher than `from`.
-	/// The logic is structured to be consistent with ready_blocks().
-	pub fn first_ready_block_header(&self, from: NumberFor<B>) -> Option<B::Header> {
-		let mut prev = from;
-		for (&start, range_data) in &self.blocks {
-			if start > prev {
-				break
-			}
-
-			match range_data {
-				BlockRangeState::Complete(blocks) => {
-					let len = (blocks.len() as u32).into();
-					prev = start + len;
-					if let Some(BlockData { block, .. }) = blocks.first() {
-						return block.header.clone()
-					}
-				},
-				_ => continue,
-			}
-		}
-		None
-	}
-
 	pub fn clear_queued(&mut self, hash: &B::Hash) {
 		if let Some((from, to)) = self.queued_blocks.remove(hash) {
 			let mut block_num = from;