Kill 'newmessageslink' and 'newmessagesdifflink' messages

Superseded ages ago by 'newmessageslinkplural' and 'newmessagesdifflinkplural'.

Also updated any docs that still referred to them.

Change-Id: Id4d99270acabbd7d72b877a091a6e3015d616826
Merge "Kill 'newmessageslink' and 'newmessagesdifflink' messages"