Remove obsolete UTC function and fix sign error
Remove obsolete UTC function and fix sign error

There was an older pre-datapackage version of the UTC offset-by-plant
grabbing functions in the EPA CEMS transform module.

Also, pretty sure there was a sign error in the use of the UTC offsets
to convert CEMS TZ naive local timestamps to UTC.

Closes #495

catalyst-cooperative/pudl,catalyst-cooperative/pudl