RelNotes: am.threeWay does not exist (yet)

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/RelNotes/2.5.0.txt

@@ -127,8 +127,6 @@ UI, Workflows & Features
  * "git send-email" learned the alias file format used by the sendmail
    program (in a simplified form; we obviously do not feed pipes).
 
- * "git am" learned am.threeWay configuration variable.
-
  * Traditionally, external low-level 3-way merge drivers are expected
    to produce their results based solely on the contents of the three
    variants given in temporary files named by %O, %A and %B on their