Remove unused functions

Resolves: #3236

\#3236 states that some functions are defined but never used, thus this
PR remove then. While removing them, this also makes sure that there is
no trouble with:
```bash
$ poetry run pytest tests/
$ poetry run pre-commit run --all-files
```
as stated at `CONTRIBUTING.md`.

The functions removed were:

- `poetry/config/config.py`:
  + `_get_validator`

- `poetry/console/commands/config.py`:
  + `_list_setting`
  + `_get_formatted_value`

- `poetry/console/commands/run.py` :
  + `_module`

- `poetry/console/commands/self/update.py :
  + `_bin_path`

- `poetry/mixology/version_solver.py`:
  + `_excludes_single_version`

- `poetry/puzzle/provider.py`:
  + `_formatter_elapsed`

- `poetry/utils/setup_reader.py`:
  + `_is_empty_result`

By removing some of those functions Flack announced that there is no
more need to do some includes, so they were removed too:
- `poetry/mixology/version_solver.py`:
  + `from typing import Any`
  + `from poetry.core.semver import Version`
  + `from poetry.core.semver import VersionRange`
- `poetry/puzzle/provider.py`:
  + `import time`