tflite: mtk_neuron: Remove unnecessary model token calculation

This CL removes `neuron_cache_token_`, which is not actually used in any
places. Besides, this CL keeps the `options_.model_token` as-is because
it is also not used, so changing it is meaningless.

BUG=b:378633716
TEST=Can run VC

Change-Id: I15eaa1622598e47fc07cc7d13176eef43bafcfe8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/tflite/+/6074029
Commit-Queue: Shik Chen <shik@chromium.org>
Reviewed-by: Shik Chen <shik@chromium.org>
Tested-by: Tommy Chiang <ototot@google.com>
Auto-Submit: Tommy Chiang <ototot@google.com>