Remove `sessionStore` to `cryptoStore` migration path
The code to migrate from the `sessionStore` to `cryptoStore` originally appeared
in https://github.com/matrix-org/matrix-js-sdk/pull/584 (2017-12-06). At this
point, it seems safe to assume most sessions that need migrating have already
done so. Removing this code simplifies store handling and removes the
`sessionStore` from most places in JS SDK.
