Minor simplifications to trmm, trsm macro-kernels.

Details:
- Simplified some code that would have allowed the diagonal of a trmm
  or trsm triangular matrix to intersect the short end of a micro-panel.
  This is disallowed via higher-level constraints on cache blocksizes, so
  this code was never needed and only served to obfuscate.
- Updated some comments in trmm, trsm macro-kernels.