The isc-config.sh script was introduced before pkg-config as is a purely
historical thing.  There are two reason for removal of isc-config.sh scripts:

a) The BIND 9 libraries are now meant to be used only from BIND 9, so there's no
   reason to provide convenience script to link with the libraries.

b) Even if that was not the case, we should and would replace the isc-config.sh
   with respective pkg-config (.pc) file for every library.