Remove stale Vulkan renderer compile-time feature flags

[cursor]

Summary

Removes three classes of stale compile-time flags in the Vulkan renderer where the active path has been fully rolled out for years. No runtime cvar behavior changes.

Branch: cursor/feature-flag-cleanup-03ad → main

Flags removed

Flag	Why safe to delete
USE_VK_PBR #else branch (vk_shader_modules.c)	USE_VK_PBR is unconditionally #defined in tr_local.h. The non-PBR shader module init path (~120 lines) was dead code; PBR generated shaders via vk_bind_generated_shaders() is the only live path.
USE_VBO_GRID ifdefs (vk_vbo.c, tr_surface.c, tr_local.h)	Always enabled whenever USE_VBO is on (also unconditional). Unwrapped #ifdef gates; grid VBO estimate/build path is the only compiled behavior.
USE_TESS_NEEDS_NORMAL / USE_TESS_NEEDS_ST2 (renderer tessellation)	Macros were commented out in tr_local.h, so optimization gates were disabled. Removed dead DetectNeeds(), shader.needsNormal/needsST2, tess fields, and conditional copy guards. Parity: with flags off, normals/ST2 were always copied; that behavior is preserved.

Behavioral parity checks

• ./scripts/compile_engine.sh vulkan core — full build succeeds; idtech3_vulkan.so links cleanly
• ./tests/scripts/test_vulkan_regression_source_guards.sh — PASS
• Renderer sources compile with no new errors (warnings unchanged)
• ./scripts/smoke_test.sh — pre-existing q3_openarena_compat_check.sh failure (r_forwardPlusDepthCull default); unrelated to this diff

Files touched

• renderers/vulkan/vk_shader_modules.c
• renderers/vulkan/vk_vbo.c
• renderers/vulkan/tr_surface.c
• renderers/vulkan/tr_local.h
• renderers/vulkan/tr_shader.c
• renderers/vulkan/tr_shade.c
• renderers/vulkan/tr_main.c
• renderers/vulkan/tr_animation.c

Candidates not changed (low confidence)

• Runtime cvars: r_renderMode, r_forwardPlus, r_vbo, r_pbr, r_volumetricFog, RTX toggles
• Broader USE_VK_PBR ifdef unwrapping across vk.h / pipeline code (still used for optional PBR fields)
• Legacy no-op cvars r_allowSoftwareGL / r_glDriver (mod cfg compatibility)