app_crypto: macro improvements (#14263)

* app_crypto: macro improvements

During `app_crypto`  macro expansion the `cfg` feature gate was injected
  into the macro client crate. This resulted in compilation error if
`serde` or `std` was not defined in client crate. This PR fixes this
problem.

For reference, the error was:

```
  error: cannot find macro `format` in this scope
    --> /home/miszka/parity/10-genesis-config/substrate-2/primitives/consensus/aura/src/lib.rs:32:3
     |
  32 |         app_crypto!(sr25519, AURA);
     |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
...

  error[E0433]: failed to resolve: use of undeclared type `String`
    -->
/home/miszka/parity/10-genesis-config/substrate-2/primitives/consensus/aura/src/lib.rs:32:3
     |
  32 |         app_crypto!(sr25519, AURA);
     |         ^^^^^^^^^^^^^^^^^^^^^^^^^^ use of undeclared type `String`
```

* app_crypto: cleanup