Stop casting CWB likelihood to an integer

From what I can gather, there was a time when likelihood and gpstime were integers for CWB events, and so there was an explicit casting of them to integers. Branson removed gpstime, but left in likelihood before during ER6. Oops.

https://git.ligo.org/lscsoft/gracedb/blame/master/gracedb/events/translator.py#L404