Remove or refactor WorkspaceDiagnosticsHandler

[snjeza]

Typescript LS reports errors only for opened files. We can do something similar if we remove WorkspaceDiagnosticsHandler. The errors for opened files are reported in DocumentLifeCycleHandler.performValidation().
If we want to keep WorkspaceDiagnosticsHandler, we can remove the POST_CHANGE event because the workspace's autobuilding property is set to true and all the errors are reported with the POST_BUILD event.
Currently, all the errors are being reported twice.
That will improve performance when using a bigger project.

@fbricon @gorkem WDYT?

[fbricon]

Keeping workspace wide errors makes sense, but I reckon error reporting can be quite slow. I'd be happy to see a better performing version of it.
So make a PR and we'll see how it fares.

[gorkem]

+1. We may actually have POST_CHANGE in there by mistake. In the early days the builds were not automatic.

[fbricon]

I'm all for it as long as we don't introduce regressions