chore: remove stale Vulkan compile-time feature flags

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Summary

Removes dead compile-time feature-flag branches in the Vulkan renderer where rollout is complete (USE_VK_PBR always on, USE_VBO_GRID always on, USE_TESS_NEEDS_* permanently disabled). Net −337 LOC; active rendering paths unchanged.

Flags removed

Flag	Why safe
USE_VK_PBR #else paths	#define USE_VK_PBR is unconditional in tr_local.h; non-PBR shader init/teardown was unreachable dead code
USE_VBO_GRID	Always enabled with USE_VBO; grid surfaces always use the VBO path — guards were no-ops
USE_TESS_NEEDS_NORMAL / USE_TESS_NEEDS_ST2	Macros commented out in tr_local.h; guarded blocks were compiled out; tess needsNormal/needsST2 plumbing removed
VERTEXSIZE 8 fallback	PBR 11-float vertex layout is the only live path when USE_VK_PBR is on

Runtime toggles preserved: vk.pbrActive, r_pbr, etc.

Files touched

• renderers/vulkan/vk_shader_modules.c — drop legacy manual shader table
• renderers/vulkan/vk_shutdown.c — drop legacy module destroy loops
• renderers/vulkan/vk_vbo.c, tr_surface.c — unwrap grid VBO + tess guards
• renderers/vulkan/tr_shade.c, tr_animation.c, tr_main.c, tr_local.h — tess flag cleanup

Test plan

• ./tests/scripts/test_vulkan_regression_source_guards.sh
• ./scripts/compile_engine.sh vulkan core
• ./scripts/smoke_test.sh ./build-vk-Release (CI)
• Manual client with mod data (GPU/display required)

Behavioral parity checks

• PBR shader binding still via vk_bind_generated_shaders() (only live init path)
• Grid BSP surfaces still routed through RB_SurfaceGridEstimate / VBO bake
• World face tessellation uses 11-component VERTEXSIZE layout
• vk.pbrActive runtime checks unchanged (compile guards removed only)

Not in scope (low confidence)

• Broader #ifdef USE_VK_PBR unwrap across remaining vk_*.c / tr_*.c
• Legacy client cvars r_allowSoftwareGL / r_glDriver (config compat)
• Runtime latched cvars: r_forwardPlus, r_renderMode, r_pbr, RTX toggles