Remove error_in_core feature #799
Description
Error should now be implemented fully in core, so there should be no need for this feature anymore.
- Remove https://github.com/scs/substrate-api-client/blob/master/src/lib.rs#L18
- Remove distinction between std::error and core::error (e.g. remove https://github.com/scs/substrate-api-client/blob/master/src/api/error.rs#L26-L29)
Related issue: rust-lang/rust#103765