Don't implement by default custom language client API

[angelozerr]

When a LSP client consumes the last version of Qute LS, it doesn't know which new API methods that it needs to implement because all API are implemented by default which return a CompletableFuture.completableFuture(null).

We need to remove default implementation to force all LSP client (like IJ Quarkus, JBoss Tools Quarkus) to implement new methods.

//cc @sbouchet

[angelozerr]

Thanks so much @datho7561 for your review, I need to do the same thing for MicroProfile LS.