fix(downloader): remove optimization for single full block download

[rkrasiuk]

Description

Remove optimization for a single full block download where the block body request was dispatched immediately.

1. The check here should've been inverse
   

   reth/crates/interfaces/src/p2p/full_block.rs

   Lines 85 to 88 in f5ce869

   	// Optimization: if we only want one block, we don't need to wait for the headers request
   	// to complete, and can send the block bodies request right away.
   	let bodies_request =
   	if count == 1 { None } else { Some(client.get_block_bodies(vec![hash])) };

2. If the block body response arrives before the header response, it will be discarded here
   

   reth/crates/interfaces/src/p2p/full_block.rs

   Lines 397 to 401 in f5ce869

   	fn insert_body(&mut self, body_response: BodyResponse) {
   	if let Some(header) = self.pending_headers.pop_front() {
   	self.bodies.insert(header, body_response);
   	}
   	}