Clean up the "installdirs" target in Makefile.in

The isc-config.sh script was removed in 2019 [1], so the top-level Makefile does not need to create ${bindir} and ${mandir}/man1 any more.

All named options creating files in the ${localstatedir}/run/named directory are able to automatically create that directory (as well as its containing directories), so there is no need for the top-level Makefile to create ${localstatedir}/run.

Clean up the "installdirs" target in the top-level Makefile accordingly.

[1] see commit 4b44351e

Closes #2709 (closed)