doc/emacs: drop domain label

This makes the generate info files somewhat less verbose, so that they
say e.g. "Key Binding" instead of "Emacs Key Binding"

doc/elisp.py

@@ -302,7 +302,7 @@ class EmacsLispDomain(Domain):
     """A domain to document Emacs Lisp code."""
 
     name = 'el'
-    label = 'Emacs Lisp'
+    label = ''
 
     object_types = {
         # TODO: Set search prio for object types