Remove ineffective code for writing more TER fields to pdb files

Remove ineffective code for writing more TER fields to pdb files

A weakly typed string containing a residue type name was passed to a
function expecting a residue name. This has led to fewer TER records
being output than expected by the author of ff9a9692e1f4. The intent
then was presumably to refer to the residue type of the previous
chain, but in fact the residue type of that residue type was used,
which is always "Other". Thus no extra TER fields were ever written.

The intended feature for PDB writing is not clearly vital, and has
never worked, so is now removed. These days, it would only be active
if pdb2gmx wrote .pdb output, since in no other case do we build chain
numbers.

Change-Id: I4e5ed09171772e17fd0fc11aff239bf7aaa966eb