chore(render): remove permanently-disabled renderer branches

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Summary

Removes dead conditional branches that were gated by if (0 && …) in Vulkan and OpenGL renderers. These were never reachable at runtime.

Flags removed

Not traditional named feature flags or cvars. Effective toggles removed:

Behavior	Previous gate	Notes
Lightmap intensity debug visualization (r_lightmap == 2 path in R_ProcessLightmap)	if (0 && r_lightmap->integer == 2)	Unreachable; active path was always the merge/legacy lightmap copy
Optional color buffer clear tied to r_fastsky in RB_BeginDrawingView	if (0 && r_fastsky->integer && …)	Unreachable
RB_DrawSun after draw-surfaces	if (0 && r_drawSun->integer)	Already documented as disabled

Cvars r_lightmap, r_fastsky, and r_drawSun remain registered for other code paths (e.g. flat sky, lightmap-only views).

Why this is safe

• Every removed branch was compile-time false (0 && …), so execution always followed the else/fall-through that remains.
• No change to map loading, shader selection, or sky/lightmap behavior that was actually running before.

Behavioral parity checks

• ./scripts/compile_engine.sh vulkan — success
• ctest in build-vk-Release — 19/19 tests passed (smoke, renderer regression check, unit tests)

Candidates not changed (low confidence / out of scope)

• CMake USE_* options (Opus, curl, optional codecs): still meaningful build toggles, not “rolled out.”
• Renderer cvars such as r_forwardPlus: documented optional scaffolding, default off — removing would change supported behavior.