Bug 1805401: Disable MOZ_NEW_XULSTORE in nightly. r=NeilDeakin

Just disable in the build config for now. It is unlikely that any issues will
come up since this is disabled in release but just in case this will allow us
to re-enable it quickly.

Differential Revision: https://phabricator.services.mozilla.com/D164565

toolkit/moz.configure

@@ -2525,20 +2525,6 @@ def wasm_sandboxing_config_defines():
 wasm_sandboxing_config_defines()
 
 
-# new XULStore implementation
-# ==============================================================
-
-
-@depends(milestone)
-def new_xulstore(milestone):
-    if milestone.is_nightly:
-        return True
-
-
-set_config("MOZ_NEW_XULSTORE", True, when=new_xulstore)
-set_define("MOZ_NEW_XULSTORE", True, when=new_xulstore)
-
-
 # new Notification Store implementation
 # ==============================================================