Revert PREMIS event limiting SQL

This was intended to limit PREMIS events for Maildir messages, but
instead was eliminating *all* PREMIS events for certain transfer types.

The immediate issue is that transfer type can be NULL under certain
circumstances, which caused != and = comparisons to both return false.
However, even after fixing this, the resulting METS seems to have a
few fewer events.

This change may come back after further testing.

This reverts commit 24d31e0c76106e6e076c90a72bd50c91f05f9977.