Removed b_aux field from cntl nodes.

Details:
- Removed b_aux field from all control tree node definitions. This field
  was being used in certain optimizations (incremental blocking) that were
  not actually being employed within BLIS, and are probably not employed
  by others.
- Updated all _cntl_obj_create() function definitions and invocations
  according to above change.
- Retired bli_gemm_blk_var4.c, which was one such function that employed
  incremental blocking, but which was never called by BLIS itself.