Source Smartsheets: remove the start_datetime option or hide it

## Tell us about the problem you're trying to solve
This option is used when pulling data from the spreadsheet to filter out rows which were modified before this data. This seems like an easy footgun for the user to screw up setting up the connector. The most common user workflow is probably going to be: first, replicate _all_ data in the spreadsheet, then over time replicate only the data which has changed. It seems much less likely that a user would want to replicate only the rows modified since a particular date from the get-go. In fact, the user probably doesn't particularly care what exactly we do as long as the data in the destination just matches the data in the source. So I would advocate for removing or hiding this option altogether.

## Describe the solution you’d like
remove or hide this option