Remove trust item listing

* gpgme.i: Remove pointer function for gpgme_trust_item_t.
* src/core.py (class Context): Remove member functions op_trustlist_all
and op_trustlist_next.

The (never working and long deprecated) trust item API has been remove
from gpgme. For details see commit 7e6a65e6 in gpgme.

• GnuPG-bug-id: T4834