Disable oom body check for MIME mails

* src/mailitem-events.cpp (EVENT_SINK_INVOKE): Disable
safety checks.

The safety check added for T3863 triggered too often.
Even if a Body is found in OOM when we send MIME that
should not be a problem as the MIME conversion will
wkill the body.

• GnuPG-Bug-Id: T3875