libiberty: Don't update and remove did_subs field from demangler structs.
The d_info field did_subs was used for estimating the string output
size. It was no longer used when the malloc-less callback interface
was introduced in 2007 (svn r121305). But the field was still updated.
When backtracking was introduced in 2013 (svn r205292) did_subs was
also added to the d_info_checkpoint struct. But except for updating
the field it was still not used.
Since it is never used just stop updating the field and remove it
from the two structs.