Remove Sourcegraph-specific LSP extension methods for global code intel

[felixfbecker]

Cross-repo codeintel should work without Sourcegraph-specific LSP extensions. This makes our codeintel stack easier to understand and possible to swap out the language server with different community/official implementations.

Here's an example of how this can work:
sourcegraph/sourcegraph-typescript#8
sourcegraph/sourcegraph-typescript#9