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
9 jobs for <span class="ref-name">remove-mfxjoinsession</span> in 114 minutes and 21 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #76416
gst indent

00:00:29

passed #76415
manifest

00:00:20

 
  Build
passed #76417
build fedora x86_64

00:02:52

 
  Test
passed #76418
check fedora

00:15:40

 
  Full Builds
passed #76419
build android arm64 api28

00:04:52

passed #76421
build cerbero cross win32

00:43:46

passed #76569
build cerbero cross win64

00:51:32

passed #76420
build cerbero fedora x86_64

00:18:12

failed #76422
build cerbero cross win64

00:47:31