Remove stale Vulkan compile flags and legacy OpenGL cvar wiring

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Flags removed

Flag / plumbing	Why safe	Parity check
USE_VK_PBR #else branch in vk_shader_modules.c	USE_VK_PBR is unconditionally #defined in tr_local.h; non-PBR shader module path was dead code	Release build + ctest -R smoke_test
USE_VBO_GRID compile gate	Always enabled whenever USE_VBO is on (also unconditional); grid VBO path is the only active path	Release build + smoke test
USE_TESS_NEEDS_NORMAL / USE_TESS_NEEDS_ST2	Macros were commented out (never defined); cleanup keeps the prior off-path behavior (unconditional normal/ST2 copies)	Release build + smoke test
r_allowSoftwareGL / r_glDriver registration on Vulkan-only builds	CMake client target defines USE_VULKAN_API only (no USE_OPENGL_API); cvars were documented as ignored legacy stubs	test_vulkan_regression_source_guards.sh + Q3/OA compat check

Changes

• Delete ~120 lines of unreachable non-PBR shader module loading.
• Inline SF_GRID VBO handling (remove USE_VBO_GRID ifdef wrappers).
• Strip disabled tessellation need guards while preserving current runtime behavior.
• Register legacy OpenGL cvars only under USE_OPENGL_API; gate SDL GLimp usage similarly.
• Add regression guards in tests/scripts/test_vulkan_regression_source_guards.sh.

Candidates not removed (low confidence)

• Runtime toggles with real off-paths: r_forwardPlus, r_vbo, r_pbr, r_renderMode, r_volumetricFog, RTX cvars
• USE_VBO / USE_VK_PBR outer #ifdef wrappers (large refactor)
• SKIP_IDPAK_CHECK (still used in files.c and build scripts)

Validation

• ./scripts/compile_engine.sh vulkan (Release) — pass
• ctest -R "smoke_test|test_vulkan_regression_source_guards" — pass
• ./scripts/q3_openarena_compat_check.sh release — pass