Don't bypass the prepared statement in stmt.Exec()

I fixed the Query case in 59dc557f48025f2793e034756cea8062643c53ca, but
it apparently didn't go the entire way.  Using the already-prepared
statement is both less surprising, and also *faster* than running
simpleExec().

This also means that there's no need to store the query inside the
"stmt" struct anymore, so get rid of it.