krb5: Don't install gssapi.h twice

[ivy]

Summary

Currently we install /usr/include/gssapi/gssapi.h twice, once in
krb5/include/gssapi and once in krb5/lib/gssapi.

The version in krb5/include/gssapi is wrong: make searches ${.PATH} and
picks crypto/krb5/src/include/gssapi.h to install, but this is the file
which is supposed to be installed in /usr/include/gssapi.h.

The problem was masked by the fact that we install the correct
gssapi/gssapi.h later in krb5/lib/gssapi.

Remove gssapi.h and the unneeded ${.PATH} entry from krb/include/gssapi
and while here, remove the unused GSSAPI_KRB5 include group.

This change does not affect the ultimate result of installworld,
it just avoids installing the same file twice with different content.

PR:     288594

[des]

I would title this “krb5: Don't install gssapi.h twice”

[ivy]

that seems reasonable, i will rewrite the commit message to match.

[brooks]

LGTM. This is about what I was going to do, except I couldn't figure out which gssapi.h to not install.

Thanks!