cw2: Move cw-storage-plus to [dev-dependencies]

[webmaster128]

In the future there can be contracts that do not use cw-storage-plus but still require cw2. To prepare for that we should try to remove the dependency on cw-storage-plus at runtime. The crate is designed to be simple enough to use raw storage access an manual deserialization. The storage key is always 636F6E74726163745F696E666F which can be hardcoded.

Keeping cw-storage-plus in [dev-dependencies] would allow us to check if cw2 writes an cw_store_plus::Item compatible storage entry or is able to read data written by cw_store_plus::Item.