tensorflow: remove unused assets from manifest

BUG=b:163339463
TEST=cq passes

Change-Id: Ic8f558b32a5074c7f8806f909c911b60ad7d8a78
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/2987243
Tested-by: Jim Pollock <jmpollock@chromium.org>
Auto-Submit: Jim Pollock <jmpollock@chromium.org>
Reviewed-by: Stuart Langley <slangley@chromium.org>