Remove broken async support.

Added ThreadingMixIn at some point when testing different approaches
to run server on background. It would add async support which would be
awesome, but unfortunately it doesn't work well with the current
system so intercept stdout/stderr. See issue #20 that requests this
async support for more information about the problems. The issue also
contains a potential solution, but it is too big task for 1.1 release.

Author: pekkaklarck

src/robotremoteserver.py

@@ -28,12 +28,10 @@
 if sys.version_info < (3,):
     from SimpleXMLRPCServer import SimpleXMLRPCServer
     from StringIO import StringIO
-    from SocketServer import ThreadingMixIn
     from xmlrpclib import Binary, ServerProxy
     PY2, PY3 = True, False
 else:
     from io import StringIO
-    from socketserver import ThreadingMixIn
     from xmlrpc.client import Binary, ServerProxy
     from xmlrpc.server import SimpleXMLRPCServer
     PY2, PY3 = False, True
@@ -187,7 +185,7 @@ def get_keyword_documentation(self, name):
         return self._library.get_keyword_documentation(name)
 
 
-class StoppableXMLRPCServer(ThreadingMixIn, SimpleXMLRPCServer):
+class StoppableXMLRPCServer(SimpleXMLRPCServer):
     allow_reuse_address = True
 
     def __init__(self, host, port):