msdk: needn't call MFXJoinSession after MFXCloneSession

According to the MSDK documation[1], MFXCloneSession is a light-weight
equivalent of MFXJoinSession after MFXInit, so MFXJoinSession call isn't
needed in the msdk plugin, otherwise the cloned session is joined to the
parent session twice, and we will get a MFX error when closing the
parent session

example pipeline:

gst-launch-1.0 videotestsrc num-buffers=100 ! \
video/x-raw,format=NV12,width=352,height=288 ! msdkh264enc ! msdkh264dec ! \
msdkh264enc ! fakesink

Error message:

0:00:00.211948518 21733 0x5586ee741c60 ERROR                   msdk
msdk.c:148:msdk_close_session: Close failed (undefined behavior)

[1]: https://software.intel.com/en-us/node/628429#MFXCloneSession

1 files changed, 0 insertions, 6 deletions

diff --git a/sys/msdk/gstmsdkcontext.c b/sys/msdk/gstmsdkcontext.c
index b365e22c0..942578b9c 100644
--- a/sys/msdk/gstmsdkcontext.c
+++ b/sys/msdk/gstmsdkcontext.c
@@ -290,12 +290,6 @@ gst_msdk_context_new_with_parent (GstMsdkContext * parent)
     return NULL;
   }
 
-  status = MFXJoinSession (parent_priv->session, priv->session);
-  if (status != MFX_ERR_NONE) {
-    GST_ERROR ("Failed to join mfx session");
-    return NULL;
-  }
-
   priv->is_joined = TRUE;
   priv->hardware = parent_priv->hardware;
   priv->job_type = parent_priv->job_type;