Deprecate calling external dssp program

Support for DSSP 4.0 (https://github.com/PDB-REDO/dssp) has been
enabled in commit da57a1f6.
This worked if CLIBD_MON env var points to some CCP4 related
compound library. In the example of dssp shipped with Debian
bullseye, that's not the case. The observation there:
It works if applied on a PDB file downloaded from RCSB with all bells
and whistles but it doesn't work if only the fix from the above commit
is applied. As OpenStructure implements all functionality from DSSP
(secondary structure and solvent accessibility), the binding to an
external program has been deprecated. dssp.AssignDSSP still exists,
but mimics the old behaviour using OpenStructure internal implementations.