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.

libiberty/ChangeLog:

	* cp-demangle.h (struct d_info): Remove did_subs field.
	* cp-demangle.c (struct d_info_checkpoint): Likewise.
	(d_template_param): Don't update did_subs.
	(d_substitution): Likewise.
	(d_checkpoint): Don't assign did_subs.
	(d_backtrack): Likewise.
	(cplus_demangle_init_info): Don't initialize did_subs.

From-SVN: r247054