Removed redundant hemm, her2k control trees.

Details:
- Removed code that generated a control tree specifically for hemm and
  symm. Instead, the gemm control tree is now configured so that it
  works for gemm, hemm, or symm.
- Retired most her2k code, as it was not being used. (Currently, her2k is
  implemented as two invocations of herk.) I couldn't think of many
  situations where her2k variants were needed.
- Removed some older her2k code.