Remove unused mobile mode code (#3206).

Mobile mode was removed in 31e36cf.

app/logical/current_user.rb

@@ -11,7 +11,6 @@ class CurrentUser
     ensure
       self.user = old_user
       self.ip_addr = old_ip_addr
-      self.mobile_mode = false
     end
   end
 
@@ -49,14 +48,6 @@ class CurrentUser
     Thread.current[:current_ip_addr]
   end
 
-  def self.mobile_mode=(mode)
-    Thread.current[:mobile_mode] = mode
-  end
-
-  def self.mobile_mode?
-    Thread.current[:mobile_mode]
-  end
-
   def self.id
     if user.nil?
       nil