remove substrate-compat feature

[niklasad1]

subxt has a feature called substrate-compat which provides a few nice impls/features from polkadot-sdk such as:

• polkadot sdk signer
• Glue code for seamless conversions between subxt types and polkadot-sdk types such as AccountId32 and similar

This comes with the downside that subxt depends on polkadot-sdk which is brings in lots of dependencies and that subxt is dependent of the polkadot-sdk releases such as i) if there is no subxt release for a while it may use an outdated polkadot-sdk version ii) that a specific user wants to use a specific release iii) the code doesn't compile with different versions of some dependency.

In addition, subxt got a lot of issues where folks have issues with a certain polkadot-sdk release/branch and this will allow users to select which polkadot-sdk release/branch would makes this easier. Of course to some degree subxt is dependent to polkadot-sdk such new transaction extensions, signed extensions and similar things.

So the idea is that subxt will remove this feature, the user has to bring these dependencies themselves

It's important that subxt provides a nice and easy example how to do this without the subxt substrate-compat feature.