[ticket/11201] Remove MSN/WLM fields

WLM will be shutdown in March 2013. Skype is the new replacement. But as Skype
uses a different login ID and service, the values in this field are useless.
So we can safely remove the field and the links/functions we create.

PHPBB3-11201
[ticket/11201] Remove database column on update

PHPBB3-11201
Merge remote-tracking branch 'remotes/nickv/ticket/11201' into develop

# By Joas Schilling
# Via Joas Schilling
* remotes/nickv/ticket/11201:
  [ticket/11201] Remove database column on update
  [ticket/11201] Remove MSN/WLM fields