Drop samples.Dockerfile and bump TensorFlow version used for Colab notebook tests

[ScottTodd]

Filing for when I get a chance to code this...

This can be replaced with a base Dockerfile (from IREE or nod-ai, doesn't really matter) and on-demand pip installs: https://github.com/openxla/iree/blob/621c186ddc8f3db8ef5b46d92d94ce756f629827/build_tools/docker/dockerfiles/samples.Dockerfile#L7-L29

Colab is shipping TensorFlow version 2.15.0 now, so we can update that and drop this from some Colab notebooks:

%%capture
!python -m pip install --upgrade tf-nightly  # Needed for stablehlo export in TF>=2.14

This may also fix https://github.com/openxla/iree/actions/workflows/samples.yml