Retired herk_t control tree definitions, usage.

Details:
- Replaced all herk_t control tree instances and usage with that of
  gemm_t, since the two types presently have the same fields. This means
  that herk, her2k, syrk, and syr2k can simply use the gemm control tree
  as-is, just as hemm and symm have been doing for some time now.
- Retired herk_t control tree creation/initialization functions.
- Retired many _target.c and .h files into 'old' directories.