CONTRIB: move spoa_example out of the tree

As previously mentioned SPOA code has nothing to do in the haproxy core
since they're not dependent on haproxy's version. This one was moved to
its own repository here with complete history:

     https://github.com/haproxy/spoa-example
diff --git a/MAINTAINERS b/MAINTAINERS
index 878d7e3..ad9a8a0 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -131,8 +131,7 @@
 
 SPOE
 Maintainer: Christopher Faulet <cfaulet@haproxy.com>
-Files: src/flt_spoe.c, include/haproxy/spoe*.h
-Files: contrib/spoa_example, doc/SPOE.txt
+Files: src/flt_spoe.c, include/haproxy/spoe*.h, doc/SPOE.txt
 
 SSL
 Maintainer: Emeric Brun <ebrun@haproxy.com>