msdk: needn't call MFXJoinSession after MFXCloneSession

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