check_rust: remove command

[davidhewitt]

Follow-up to #129 .

Looks like check_rust command has been broken for a while anyway:

(pyo3) david@david-laptop:~/dev/setuptools-rust/examples/html-py-ever$ python setup.py check_rust
Traceback (most recent call last):
  File "/home/david/dev/setuptools-rust/examples/html-py-ever/setup.py", line 7, in <module>
    setup(
  File "/home/david/.virtualenvs/pyo3/lib/python3.9/site-packages/setuptools/__init__.py", line 153, in setup
    return distutils.core.setup(**attrs)
  File "/usr/lib/python3.9/distutils/core.py", line 134, in setup
    ok = dist.parse_command_line()
  File "/usr/lib/python3.9/distutils/dist.py", line 483, in parse_command_line
    args = self._parse_command_opts(parser, args)
  File "/home/david/.virtualenvs/pyo3/lib/python3.9/site-packages/setuptools/dist.py", line 903, in _parse_command_opts
    nargs = _Distribution._parse_command_opts(self, parser, args)
  File "/usr/lib/python3.9/distutils/dist.py", line 555, in _parse_command_opts
    raise DistutilsClassError(msg % cmd_class)
distutils.errors.DistutilsClassError: command class <class 'setuptools_rust.check.check_rust'> must provide 'user_options' attribute (a list of tuples)

[messense]

setuptools-rust/README.md

Line 148 in f2d4c47

- check - Standard check command executes cargo check for all rust

README also needs update

[messense]

I've never used check_rust before, I'm fine with removing it.

[davidhewitt]

IMO it's much easier for developers to just use cargo check themselves to do exactly what they want!