Remove obsolete Vulkan repository leftovers

[antono3]

Summary

• remove the unused legacy flags.txt Volk configuration sample
• replace the inherited Python/PyCharm .gitignore with focused editor, native-build, and setup-helper entries
• document setup.vsh as an optional download helper and distinguish it from the pinned CI setup

Verification

• git diff --check
• v fmt -verify setup.vsh
• v -check-syntax setup.vsh
• ergonomic test suite against REGISTRY_COMMIT headers and VOLK_COMMIT: 2/2 passed
• generator smoke target compiled against an isolated module assembly that omits flags.txt