security/p5-Authen-SASL: remove KERBEROS from OPTIONS_DEFAULT

security/cyrus-sasl2 does not bundle the GSS-API mech plugin by default,
for consistency reasons this port shouldn't do it too.
This is in preparation of 'uses=gssapi' knob in the ports system.

PR: 281120
Approved by: jrm (mentor), otis (mentor), perl (maintainer timeout)
Differential Revision: https://reviews.freebsd.org/D47929