CommentStore: hash() has only one caller, make private

This method seems to have only a single caller which happens to be
in the CommentStore.php class itself. Maybe we should just make this
a private utility.

Test plan
=========

If this patch fails, then I'm probably wrong and something else is
using it otherwise, then we can make it private.

Change-Id: Ib774af48ddf7e0a42a1c9a08cdecd5317e238cb4

includes/CommentStore/CommentStore.php

@@ -391,7 +391,7 @@ class CommentStore {
 	 * @param string|null $data Comment data
 	 * @return int 32-bit signed integer
 	 */
-	public static function hash( $text, $data ) {
+	private static function hash( $text, $data ) {
 		$hash = crc32( $text ) ^ crc32( (string)$data );
 
 		// 64-bit PHP returns an unsigned CRC, change it to signed for