Don't add default dependencies if none exist

[fbertsch]

Currently, we add the default Glean dependency if none exist. However this runs contrary to expectations; we now have tables for e.g. firefox-ios with Android-specific fields.

We should instead allow there to be no dependencies.

This change will be backwards-incompatible with existing tables, since it will drop fields. We should be able to deal with this by using a SELECT * EXCEPT on the existing data. Deploying this change will require coordination with ops, as described in this bug.

cc @mdboom @Dexterp37

[Dexterp37]

Ouch :D

[fbertsch]

Since we're simply dropping columns, we could manually remove these columns from the views instead. The downside being we need to keep the columns during dependency generation (in some way), but the upside around deploy may be worth it.