Remove TWOPASS_CN_PROC guard now that the two-pass path is the only
non-unrolled option:

  - Comment out #define UNROLL_CN_PROC 1 so the two-pass path is
    active by default on all targets (AVX2, AVX512, aarch64/NEON).
  - Collapse the four dispatch sites from
      #ifndef UNROLL_CN_PROC
        #ifdef TWOPASS_CN_PROC ... #else ... #endif
      #else  <unrolled switch>
    to the simpler
      #ifndef UNROLL_CN_PROC
        nrLDPC_cnProc_BG{1,2}_2pass(...)
      #else  <unrolled switch>
  - The unrolled path (UNROLL_CN_PROC) is retained for benchmarking
    and will be removed in a follow-up once the two-pass path is
    confirmed as the permanent default.

Tested on: AMD Ryzen (AVX2), Intel (AVX512), Rockchip A76 (NEON),
           NVIDIA Neoverse (DGX Spark, GH200).
Signed-off-by: Raymond Knopp <raymond.knopp@eurecom.fr>
Assisted-by: Claude:claude-sonnet-4.6