Remove redundant __contains__ entry from Message docs.

Doc/library/email.message.rst

diff --git a/Doc/library/email.message.rst b/Doc/library/email.message.rst
index cf02531..f261b9c 100644
--- a/Doc/library/email.message.rst
+++ b/Doc/library/email.message.rst
@@ -208,12 +208,6 @@
       headers.
 
 
-   .. method:: Message.__contains__(name)
-
-      Return true if the message contains a header field named *name*, otherwise
-      return false.
-
-
    .. method:: keys()
 
       Return a list of all the message's header field names.