Drop mr.developer usage.

It is not used by default.

It mangles buildout:develop as described in https://github.com/fschulze/mr.developer/issues/57,
and buildout:develop is used in many project to checkout and develop various
eggs in "slapos" way.

stack/erp5.cfg

@@ -3,7 +3,6 @@ extensions =
   slapos.rebootstrap
   slapos.zcbworkarounds
   buildout-versions
-  mr.developer
 
 find-links =
     http://www.nexedi.org/static/packages/source/slapos.buildout/