Add note on removed upgrade hooks.

CHANGELOG.txt

@@ -8,6 +8,7 @@ Feeds 7.x 2.0 XXXXXXXXXXXXXXXXXXX
   batching.
 - #928728: Track source states by stage, not by plugin. Note: call signature of
   FeedsSource::state() has changed.
+- Remove 6.x upgrade hooks.
 - #923318: Fix Fatal error: Call to a member function import() on a non-object
   occuring on cron.
 - Clean up basic settings form.