Remove the terminal option from config.

The welcome message is displayed using xmessage(1), not using your
terminal. Thus, it makes no sense to have this option anymore. Also,
the new lex/yacc parser cannot correctly handle the situation:
normal variables are expanded before parsing the file. As a replacement,
you can use:
  set $terminal /usr/bin/urxvt

docs/userguide

@@ -178,10 +178,6 @@ and edit it with a text editor.
 
 === General configuration
 
-terminal::
-	Specifies the terminal emulator program you prefer. It will be started
-	by default when you press Mod1+Enter, but you can overwrite this. Refer
-	to it as +$terminal+ to keep things modular.
 font::
 	Specifies the default font you want i3 to use. Use an X core font
 	descriptor here, like