Remove unneeded migrations

[gazayas]

These migrations can be found 1-for-1 in the starter repository, so I don't think we need them here in the core repository.

I did not delete migrations for bullet_train-outgoing_webhooks because we need those for the tests.

Besides that I almost skipped over bullet_train-integrations-stripe because I thought we would need the migrations, but I was thinking of bullet_train-billing-stripe (The following is from the documentation).

2.3. Copy Database Migrations

Use the following two commands on your shell to copy the required migrations into your local project:

cp `bundle show --paths | grep bullet_train-billing | sort | head -n 1`/db/migrate/* db/migrate
cp `bundle show --paths | grep bullet_train-billing-stripe | sort | head -n 1`/db/migrate/* db/migrate

Totally understandable if this is too much in one PR, just let me know if you want me to split it up or take another approach.