Remove custom ISC SPNEGO implementation (v9.16)

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.

This MR removes the custom ISC SPNEGO implementation, and removes the option from both autoconf and win32 Configure script. Unknown options are being ignored, so this doesn't require any special handling.

Closes #2607 (closed)