tests/exynos: remove connector_find_plane

No test uses DRM planes at the moment so this function
is never called. Inspection of the git history shows
that DRM planes were also never used in these tests
in the past.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Acked-by: Emil Velikov <emil.l.velikov@gmail.com>