remove substrate compat

[niklasad1]

Close #1847

[jsdw]

Just repeating what I already said on the call: I'd either:

1. Remove this example, or
2. Show in the fn main() for it something that demonstrates when this approach is good to take (ie when would I copy this example versus copying the simpler substrate_compat_signer one which ultimately runs the same code)

[jsdw]

I like this example; for me it shows that it's not hard to use Substrate's signer stuff instead if people would rather do that!

[jsdw]

Based on the signer example I feel happy enough removing the substrate compat feature; it's not too hard plugging things like Substrate's signer stuff in, and this means we are independent and one can write SUbxt code against any version of polkadot-sdk that they want :)

[jsdw]

Whoops; my suggestion broke the cargo check; I forgot about the extra import needed!