Remove these from the registry after initialization, the vaapi
one could just cause undesired output/warnings after seccomp forbid
it to do something. The bmcdec element is dumber and would just
crash after shmget() despite trying to refuse it politely with an
appropriate errno.

Fixes SIGSYS on shmget() from gstreamer1-plugin-crystalhd if it
happens to be installed and a video file is indexed. The blacklisting
code has been lifted from totem-video-thumbnailer.