[DBD-SQLite] Removing the on-by-default referential integrity.

[Adam Kennedy]

For the first production release of DBD::SQLite with foreign keys,
it's starting to make me nervous that we will enable it by default.

As things currently stand, nobody that is using SQLite has ever seen
this feature before. They haven't had the chance to work with it at
all before we shove it down their throats.

I think I'd like to follow SQLite itself for now and default it off.

Thoughts?

Adam K