util-hashing: Remove deprecation from KeyHasher

Motivation

`c.t.hashing.KeyHasher` was marked deprecated but it has remained
heavily in use. The API is rather small and harmless and the benefits
of migration are nebulous.

Solution

Remove the deprecation annotation.

RB_ID=800225

util-hashing/src/main/scala/com/twitter/hashing/KeyHasher.scala

@@ -2,8 +2,9 @@ package com.twitter.hashing
 
 /**
  * Hashes a key into a 32-bit or 64-bit number (depending on the algorithm).
+ *
+ * @see the companion object for common implementations.
  */
-@deprecated("Prefer Hashable[Array[Byte],Long]", "6.12.1")
 trait KeyHasher {
   def hashKey(key: Array[Byte]): Long
 }