CDB-4182 CSV Import guessing

https://cartodb.atlassian.net/browse/CDB-4182
It did (here at the Importer) part of what now the guessing does properly: Casting the "postedtime" column to date.

With ogr2ogr2 is not needed, and was only used if from Twitter datasource, so is just removing it from here.

It is still used from Column typecasting from the editor so file cannot be removed.