do not include the userdata campaigns folder in editor,

...seems to be included elsewhere already and causes duplicates. Bug
spotted by shadowmaster.

data/_main.cfg

@@ -14,9 +14,6 @@
 [textdomain]
     name="wesnoth-editor"
 [/textdomain]
-# This enables UMC to define their custom terrains inside an EDITOR ifdef,
-# so the terrains become available in the editor by default
-{~campaigns/}
 #endif
 {scenario-test.cfg}
 {scenario-formula.cfg}