baseparse: Don't do buffer dropping in passthrough mode

Passthrough is supposed to pass buffers through, so don't clip them then, especially as the subclass might not even have called gst_base_parse_set_frame_rate in passthrough mode to set up the lead_in_ts and lead_out_ts private member variables. This is common, as it's usually done in their handle_frame implementation, which doesn't even get called in passthrough push mode.