gpg: avoid chatter about trustdb when --quiet

* g10/trustdb.c (tdb_check_trustdb_stale): avoid log_info() when
  opt.quiet

gpg(1) says:

-q, --quiet
       Try to be as quiet as possible.

While the mentions about the stale trustdb information are edifying,
they aren't necessary, and shouldn't be emitted when the user requests
--quiet.

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>