Drop all code related to generating CLIs.

All of the generated CLI code depends on google-apputils, which is a
python2.7-only library. Given that no one is directly using the generated CLIs
right now, there's no point in porting it to (say) argparse.

Fixes #8.

The bulk of the change here is just deletion; I suspect there are now more
dangling references, so some tree-shaking/dead-code-elimination could be
fruitful. Other changes:

* the client generation test now ensures each client can be imported, as
  opposed to invoking `<generated_cli> --help`.
* the samples and sample generating script drop the CLIs.

Note that I *did* keep the flag for `gen_client`, so that any existing code
passing `--nogenerate_cli` wouldn't be broken.
19 files changed