Removed extra indexing on schema removal

[dogversioning]

This PR removes a second instance of array indexing in the base_table_builder, which was causing us to search only the first character of a potential string for a name like "schema"."table"

This is currently only an issue when we are creating a completely empty table.

Checklist

• [X] Consider if documentation (like in docs/) needs to be updated
• [X] Consider if tests should be added
• [X] Run pylint if you're making changes beyond adding studies
• [X] Update template repo if there are changes to study configuration

[dogversioning]

Since this is affecting sites right now, I'm electing to not write a unit test this second, but am tracking the need for catching this case at https://github.com/smart-on-fhir/cumulus-library/issues/120