Remove custom ISC SPNEGO implementation

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.

This commit 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.