removing MK_GSSAPI

hello,

i've posted a review to remove the WITHOUT_GSSAPI src.conf knob in 15.0:
https://reviews.freebsd.org/D51859.  my rationale for this is that since
this is already a no-op for MIT Kerberos, there's no real need to keep
it around for legacy Heimdal and removing it simplifies the build logic
a little, since you either have Kerberos or you don't.

Cy suggested that we might instead want to modify MIT Kerberos to make
WITHOUT_GSSAPI do something.  i'm not sure this is useful, because
without GSSAPI you can't really use Kerberos for anything even if you
build it; in particular, both ssh and gssd require GSSAPI.  in the past,
this knob might have made sense since base gssapi was separate from the
implementation (Heimdal), but with MIT Kerberos, this is no longer true.

but, perhaps there's a reason to keep this knob around?