Remove custom ISC SPNEGO implementation
The custom ISC SPNEGO mechanism implementation is no longer needed on the basis that all major Kerberos 5/GSSAPI (mit-krb5, heimdal and Windows) implementations support SPNEGO mechanism since 2006.
The change has been already implemented since BIND 9.17.2, but never properly documented in the release notes. This MR adds a CHANGES and release note for the ISC SPNEGO removal.
Closes #2607 (closed)
Edited by Ondřej Surý