nip13: remove section about pow prefix querying.

prefix searching was removed from the spec.

13.md

@@ -103,16 +103,6 @@ function countLeadingZeroes(hex) {
 }
 ```
 
-Querying relays for PoW notes
------------------------------
-
-If relays allow searching on prefixes, you can use this as a way to filter notes of a certain difficulty:
-
-```
-$ echo '["REQ", "subid", {"ids": ["000000000"]}]'  | websocat wss://some-relay.com | jq -c '.[2]'
-{"id":"000000000121637feeb68a06c8fa7abd25774bdedfa9b6ef648386fb3b70c387", ...}
-```
-
 Delegated Proof of Work
 -----------------------