DOC/CLEANUP: lua-api: removing duplicate date functions doc

As reported by Thierry, core.asctime_date() and core.rfc850() were
both documented multiple times in lua-api doc.

This does not need to be backported.
diff --git a/doc/lua-api/index.rst b/doc/lua-api/index.rst
index 847ab52..2f73f35 100644
--- a/doc/lua-api/index.rst
+++ b/doc/lua-api/index.rst
@@ -460,24 +460,6 @@
 	Sun Nov  6 08:49:37 1994
 ..
 
-.. js:function:: core.rfc850_date(date)
-
-  **context**: body, init, task, action
-
-  This function take a string representing http date, and returns an integer
-  containing the corresponding date with a epoch format.
-
-  :param string date: a date http-date formatted
-
-.. js:function:: core.asctime_date(date)
-
-  **context**: body, init, task, action
-
-  This function take a string representing http date, and returns an integer
-  containing the corresponding date with a epoch format.
-
-  :param string date: a date http-date formatted
-
 .. js:function:: core.msleep(milliseconds)
 
   **context**: body, init, task, action