MDL-64985 message: deprecate unused strings

lang/en/deprecated.txt

@@ -153,3 +153,4 @@ hidedockpanel,core_block
 undockall,core_block
 undockblock,core_block
 undockitem,core_block
+canceledit,core_message

lang/en/message.php

@@ -38,7 +38,6 @@ $string['blockuser'] = 'Block user';
 $string['blockuserconfirm'] = 'Are you sure you want to block {$a}?';
 $string['blockuserconfirmbutton'] = 'Block';
 $string['blocknoncontacts'] = 'Prevent non-contacts from messaging me';
-$string['canceledit'] = 'Cancel editing messages';
 $string['cancelselection'] = 'Cancel message selection';
 $string['contactableprivacy'] = 'Accept messages from:';
 $string['contactableprivacy_onlycontacts'] = 'My contacts only';
@@ -279,3 +278,4 @@ $string['outputdisabled'] = 'Output disabled';
 $string['outputdoesnotexist'] = 'Message output does not exist';
 $string['outputenabled'] = 'Output enabled';
 $string['outputnotconfigured'] = 'Not configured';
+$string['canceledit'] = 'Cancel editing messages';