DOC: config: remove duplicated "http-response sc-set-gpt0" directive

This directive was erroneously duplicated.

This patch could be backported as far as 2.5.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 8b2b9a6..c5e6d2a 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -7927,8 +7927,6 @@
                                         [ { if | unless } <condition> ]
 http-response sc-set-gpt0(<sc-id>) { <int> | <expr> }
                                    [ { if | unless } <condition> ]
-http-response sc-set-gpt0(<sc-id>) { <int> | <expr> }
-                                   [ { if | unless } <condition> ]
 
   These actions set the 32-bit unsigned General Purpose Tags according to the
   sticky counter designated by <sc-id>. Please refer to "http-request