msdk: don't reset the external frame allocator

In gst-msdk, a mfx session may be shared between different gst
elements, each element tries to set the frame allocator. However, per
the MSDK documation[1], the behavior is undefined if reset the frame
allocator while the previous allocator is in use. Fortunately all
elements use the same frame allocator, so we can avoid to call
MFXVideoCORE_SetFrameAllocator again.

[1]: https://software.intel.com/en-us/node/628430#MFXVideoCORE3
6 jobs for fix-frame-allocator in 32 minutes and 12 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed #75532
gst indent

00:00:29

passed #75531
manifest

00:00:18

 
  Build
passed #75533
build fedora x86_64

00:03:07

 
  Test
passed #75534
check fedora

00:15:33

 
  Full Builds
passed #75535
build android arm64 api28

00:02:54

passed #75536
build cerbero fedora x86_64

00:13:02