Remove RUNSUB for GROUP (#1047)

This is not supported by MapServer. See https://mapserver.org/cgi/runsub.html#parameters-supported.

Co-authored-by: Seth G <sethg@geographika.co.uk>

Author: github-actions[bot]

en/mapfile/class.txt

@@ -125,9 +125,6 @@ GROUP [string]
         ...
       END # layer
 
-    .. note::
-        |RUNSUB|
-
 .. index::
    pair: CLASS; KEYIMAGE
    :name: mapfile-class-keyimage