Feature-flag cleanup: remove stale USE_BANS, USE_TESS_NEEDS_*, USE_STB_TRUETYPE

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Summary

Removes stale feature flags and dead gated branches that were already inactive or never wired into the build. No intentional behavior change on the active code paths.

Flags removed

Flag / cvar	Why safe to delete
USE_STB_TRUETYPE	Passed from compile_engine.sh and CI but never referenced in CMakeLists.txt — pure ghost CMake flag.
USE_BANS	Preprocessor macro was never defined anywhere; ~650 lines of ban commands, file I/O, and connect-time checks were dead code. Auth-server banUser/banClient hooks were inside the same never-compiled block.
USE_TESS_NEEDS_NORMAL / USE_TESS_NEEDS_ST2	Defines in tr_local.h were already commented out, so all conditional branches were inactive. Tessellation already followed the always-on copy path (normals + ST2).
USE_OPENVDB	Never defined at build time. VDB loader is NanoVDB-only; removed misleading "compile with USE_OPENVDB" startup/failure messaging.
cg_stereoSeparation	Deprecated ROM cvar with no remaining consumers; registration removed from client init.

Behavioral parity checks

• ./scripts/compile_engine.sh vulkan — Release build succeeded (GCC/Clang path via default script)
• ./scripts/smoke_test.sh — 17/17 passed (binary checks, server startup, shader validation, compatibility guards)
• Server ban/connect path unchanged (ban system was never compiled in)
• Renderer tessellation unchanged (active path was already unconditional)

Follow-up candidates (not in this PR — lower confidence or larger scope)

• Flatten always-on USE_VK_PBR / USE_VBO / USE_FOG_COLLAPSE defines in tr_local.h (~30 renderer files)
• Remove USE_VULKAN=OFF CMake option (already fatal-errors)
• Deprecate runtime cvars with live 0 branches (r_forwardPlus, r_fbo, etc.) per docs/DEPRECATION_POLICY.md