nit: remove a bad comment line by merging mistake (#4942)

substrate/primitives/trie/src/lib.rs

@@ -91,7 +91,6 @@ pub type PlainDB<'a, K> = dyn hash_db::PlainDB<K, trie_db::DBValue> + 'a;
 /// key conflict for non random keys).
 pub type PrefixedMemoryDB<H> = memory_db::MemoryDB<H, memory_db::PrefixedKey<H>, trie_db::DBValue>;
 /// Reexport from `hash_db`, with genericity set for `Hasher` trait.
-/// This uses the `KeyFunction` for prefixing keys internally (avoiding
 /// This uses a noops `KeyFunction` (key addressing must be hashed or using
 /// an encoding scheme that avoid key conflict).
 pub type MemoryDB<H> = memory_db::MemoryDB<H, memory_db::HashKey<H>, trie_db::DBValue>;