Remove documentation of "Edit Message" feature

The "Edit Message" button was removed from the Change Screen; the
commit message is now edited in the inline editor.

Remove the documentation of "Edit Message" from the Review UI
documentation.

Note that some screen shots still show the "Edit Message" button
and these will need to be updated too.

Change-Id: I35f74b9c28cacda39e801269d39d42be5ba94030
2 files changed