This was broken even before 0649e637 and was always expanding the
`Image`, not the movie clip (even if the source was set to
`CAM_BGIMG_SOURCE_MOVIE`)

Now the rule here seems to be to always expand unconditionally, so
remove checking the source and always expand image and movie clip.

Co-authored-by: Philipp Oeser <philipp@blender.org>
Pull Request #104815