Only foreground censor clustered triggers, not pre-cluster ones

[tdent]

In pycbc_coinc_statmap, it seems like an error to make foreground censor veto windows on events before clustering zerolag (https://github.com/gwastro/pycbc/blob/master/bin/all_sky_search/pycbc_coinc_statmap#L142).

At least, since the pre-clustered events are not visible and not part of any results there doesn't seem to be any reason to veto their times, and it risks significantly (and spuriously) reducing the remaining time available for injections to be searched.