Remove Carer's Allowance backdrop stubs

I don't think anybody's used these in a long time. Rather than update
them to reflect data changes, it's easier to remove them.
Standardise "transactions-by-channel" config

Stagecraft/Backdrop are having a schema added to enforce some field
names, so the lasting power of attorney and carer's allowance
data sets are being migrated to use this new config.

This change is a reflection of that migration.
Merge pull request #678 from alphagov/73802338-transactions-by-channel-schema

Standardise "transactions-by-channel" config