Remove custom SQLITE_UNCLOSED error code.

This error code was introduced at some point to help track when
a database could not be closed because a statement was not finalized.
Now that the DB wrappers have been rewritten, it is technically no longer
possible for this to happen, so we can remove this.

Change-Id: Ia3c8167a0af522c852adbbd15c83c3c8f5732b26