bitbake: xmlrpc: delete unused code BitBakeUIEventServer

BitBakeUIEventServer is an unused class that pushes UI
events over a separate thread.

The current version of XMLRPC server works just fine with
the classic UI event handlers, so this class is not needed.

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>