Revert "fix session compatibility"

This reverts commit 201ba41.

Author: timwr

lib/msf/core/post_mixin.rb

@@ -29,8 +29,8 @@ def initialize(info={})
   #
   # @raise [OptionValidateError] if {#session} returns nil
   def setup
-    if not session
-      raise Msf::OptionValidateError.new(["SESSION"])
+    unless session_compatible?(session)
+      raise Msf::OptionValidateError.new(["SESSION (type not valid for this module)"])
     end
 
     super