Fix multiple savepointing transactions on DBD::SQLite

Fix multiple savepointing transactions on DBD::SQLite

Problem was missed during review of 86a51471ce (how the fuck did I let this
abomination through anyway), and not encountered due to insufficient testing.
The naive statement parser in DBD::SQLite when running against older libsqlite
mistakenly treats ROLLBACK TRANSACTION TO... as an actual TXN rollback, and
as a result desyncs the internal AutoCommit flag state [1]
Fix by simply using the shorter (still valid) syntax [2], and by removing the
sloppy workaround hiding the actual problem.

[1] https://github.com/DBD-SQLite/DBD-SQLite/blob/1.42/dbdimp.c#L824:L852
[2] http://www.sqlite.org/lang_savepoint.html