back-sch: do not clobber target of the pblock for idview
When extracting idview all we care is the DN of new target.
We don't really use the rewritten target as a string anymore,
so there is no need to rewrite the string in the pblock.
This fixes a bug when running with 389-ds 1.3.5.10+ which is more
strict about modification of the values in pblock.
Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1360245