Remove support for the DispatchOnConnectionQueue message attribute
https://bugs.webkit.org/show_bug.cgi?id=109743

Reviewed by Sam Weinig.

* Platform/CoreIPC/HandleMessage.h:
(CoreIPC):
* Scripts/webkit2/messages.py:
(handler_function):
(generate_message_handler):
* Scripts/webkit2/messages_unittest.py:

git-svn-id: svn://svn.chromium.org/blink/trunk@142783 bbb929c8-8fbe-4397-9dbb-9b2b20218538