Remove legacy skills dir read-support in btw 1.5.0 #204

Follow-up to the deprecation warning added for the legacy skills directory (btw <= 1.2.0 install target, R_user_dir("btw", "config")/skills).

• Delete legacy_skills_dir (R/tool-skills.R:238) from default_user_skill_dirs()
• Delete warn_legacy_skill_dir() (R/tool-skills.R:255-284)
• Update/remove associated tests in tests/testthat/test-tool_skills.R
• Update NEWS.md