Don't increment accepted_packets in trace_event

For formats that use the normal trace_event_device and
trace_event_trace functions to implement trace_event functionality,
this was resulting in accepted_packets being incremented twice:
once in the call to trace_read_packet within the standard event
function and once again in trace_event.

Removed the increment from trace_event and updated all formats
that provide a custom trace_event implementation to make sure they
either call trace_read_packet themselves or update the counters
properly (with the former preferred wherever possible).

Thanks to Alistair King for finding this bug.