Retired trmm_t control tree definitions, usage.

Details:
- Replaced all trmm_t control tree instances and usage with that of
  gemm_t. This change is similar to the recent retirement of the herk_t
  control tree.
- Tweaked packm blocked variants so that the triangular code does NOT
  assume that k is a multiple of MR (when A is triangular) or NR (when
  B is triangular). This means that bottom-right micro-panels packed for
  trmm will have different zero-padding when k is not already a multiple
  of the relevant register blocksize. While this creates a seemingly
  arbitrary and unnecessary distinction between trmm and trsm packing,
  it actually allows trmm to be handled with one control tree, instead
  of one for left and one for right side cases. Furthermore, since only
  one tree is required, it can now be handled by the gemm tree, and thus
  the trmm control tree definitions can be disposed of entirely.
- Tweaked trmm macro-kernels so that they do NOT inflate k up to a
  multiple of MR (when A is triangular) or NR (when B is triangular).
- Misc. tweaks and cleanups to bli_packm_struc_cxk_4m.c and _3m.c, some
  of which are to facilitate above-mentioned changes whereby k is no
  longer required to be a multiple of register blocksize when packing
  triangular micro-panels.
- Adjusted trmm3 according to above changes.
- Retired trmm_t control tree creation/initialization functions.