MP Method Selection: removed text saying registered nicknames are optional

[ci skip]

This is currently only the case on the 1.14 server, but it's easiest just to
remove the line completely rather than constantly adding or re-adding it based
on whether you're using a or dev series.

(cherry-picked from commit 6f057d8b02)

data/gui/window/mp_method_selection.cfg

@@ -90,7 +90,6 @@
 									max_input_length = 20
 
 									tooltip = _ "You may login to the official multiplayer server with your forum username and password.
-Note that registration is not required to join the server: you need to supply a password only if you wish to use a registered username.
 
 The Battle for Wesnoth forum can be found at https://forums.wesnoth.org/"
 								[/text_box]