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)