CLEANUP: validate join/cap style centrally

[brunobeltran]

PR Summary

Simple change. There was a straggling spot in lines.py that still did its own validation of joinstyle/capstyle instead of using rcsetup.validate_joinstyle/_capstyle.

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• [N/A] New features are documented, with examples if plot related.
• [N/A] Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and pydocstyle<4 and run flake8 --docstring-convention=all).
• [N/A] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [N/A] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

[timhoffm]

Might be that we want validators in a dedicated central place other than rcsetup in the future. Validators being in rcsetup is just the result of recsetup historically being the only place using validators.