ChangeScreen2: Remove "Edit Message" button

Edit Message opened a textarea, which is ugly and a lot less functional
compared to the CM4 instance available by clicking Edit, Commit Message.

Drop the special Edit Message button. To edit any part of a pending
change users should click "Edit" in the Files bar and then proceed
to modify "files" (Commit Message and/or files), then publish that
edit to make a new patch set.

Change-Id: I984e7fe99cfe42601e63b3a21dec8d0ca2bbce39
6 files changed