Clean up python configuration:

- Remove experimental attributes (local_checks, python_include, numpy_include, remote_config_repo). Use of experimental remote repo can only be enabled now via env variable (preferred to change to workspace file)
- Fix hardcoded fallback value for python bin. New fallback is to use 'which python' and fail if python not set in path (solves tensorflow/tensorflow#9436 / works around bazelbuild/bazel#3057).
- Fixes minor issues with template: adds spaces to srcs in genrule, remove extra space after genrules.
- Some dead code clean up (use of empty configuration)

PiperOrigin-RevId: 160594421