Remove "tkey-gssapi-credential" and related code

Remove "tkey-gssapi-credential" and related code

Since the "tkey-gssapi-credential" statement has been previously
deprecated, mark it as ancient and remove all code related to it:

  - The code processing the "tkey-gssapi-credential" statement in the
    configuration is the only user of the dst_gssapi_acquirecred() and
    dst_gssapi_releasecred() functions, so remove them along with their
    static helper functions and a backup definition of the
    GSS_KRB5_MECHANISM macro.

  - When calling gss_accept_sec_context(), pass GSS_C_NO_CREDENTIAL
    instead of the credential acquired by gss_acquire_cred().
    (Previously, NULL was passed when "tkey-gssapi-credential" was not
    specified.  Kerberos headers define GSS_C_NO_CREDENTIAL as
    (gss_cred_id_t) 0, so the logic was effectively the same, but using
    the GSS_C_NO_CREDENTIAL macro is more appropriate.)  This renders
    the 'cred' parameter for dst_gssapi_acceptctx() redundant, so remove
    it from the prototype of the latter.  (Contrary to what the
    documentation for dst_gssapi_acceptctx() claims,
    dst_gssapi_releasecred() does not need to subsequently be called to
    free the GSS-API context; a dst_gssapi_deletectx() call in
    gssapi_destroy() takes care of that when the dynamically generated
    TSIG key is destroyed.)

  - Remove the 'gsscred' member from struct dns_tkeyctx, along with its
    related dns_gss_cred_id_t typedef.

Update the relevant sections of the ARM and code comments accordingly.

This makes the "tkey-gssapi-keytab" statement the only way to set up
GSS-TSIG in named.

Remove redundant code from bin/named/tkeyconf.c while at it.