python: Fix a double encoding attempt on an Unicode string

python: Fix a double encoding attempt on an Unicode string

Encoding from 'unicode' to 'str' that has been added to the Stream class
in commit 2254074e3067 ("python: fix python3 encode/decode on Windows")
conflicts with SSLStream which already contains a quirk for pyopenssl
that does the same thing.

This results in a double encoding attempt when SSL is used and we crash
and burn due to:

Traceback (most recent call last):
  File "../.././test-ovsdb.py", line 874, in <module>
    main(sys.argv)
  File "../.././test-ovsdb.py", line 869, in main
    func(*args)
  File "../.././test-ovsdb.py", line 655, in do_idl
    idl_set(idl, command, step)
  File "../.././test-ovsdb.py", line 526, in idl_set
    status = txn.commit_block()
  File "/home/jkbs/src/ovs/python/ovs/db/idl.py", line 1405, in commit_block
    status = self.commit()
  File "/home/jkbs/src/ovs/python/ovs/db/idl.py", line 1388, in commit
    if not self.idl._session.send(msg):
  File "/home/jkbs/src/ovs/python/ovs/jsonrpc.py", line 540, in send
    return self.rpc.send(msg)
  File "/home/jkbs/src/ovs/python/ovs/jsonrpc.py", line 244, in send
    self.run()
  File "/home/jkbs/src/ovs/python/ovs/jsonrpc.py", line 203, in run
    retval = self.stream.send(self.output)
  File "/home/jkbs/src/ovs/python/ovs/stream.py", line 808, in send
    return super(SSLStream, self).send(buf)
  File "/home/jkbs/src/ovs/python/ovs/stream.py", line 391, in send
    buf = buf.encode('utf-8')
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2 in position 83: ordinal not in range(128)

Remove the quirk from SSLStream as the base class now does encoding.

Reported-by: Marcin Mirecki <mmirecki@redhat.com>
Signed-off-by: Jakub Sitnicki <jkbs@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>