Remove outdated modules and their tests

Remove modules that have been fully deprecated, primarily by the switch
to a Pydantic data model rather than using the raw YAML interface.

These modules included:

- check: This module has been completely replaced by Pydantic
  validation.

- simple: This is the old data model that was replaced by a Pydantic
  implementation.

- visitor: This is an implementation of a typed visitor pattern, which
  is not needed anymore.

The corresponding test cases for these modules were removed as well.

The `basic-check` command was removed from the `cli` module and the
corresponding test was removed from `test_cli` as well.

python/felis/__init__.py

@@ -20,9 +20,7 @@
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 from . import types
-from .check import *
 from .version import *
-from .visitor import *
 
 DEFAULT_CONTEXT = {
     "@vocab": "http://lsst.org/felis/",