Add quick fix for dead code.

[yaohaizh]

Fix for #437

However, there is a bug from the VSCode side: microsoft/vscode#43878

@fbricon

[fbricon]

Seems to work ok, however I find one behavior particularly annoying, and the same thing happens in Eclipse, with:

    public String getLastName() {
        if (true) {
            return "foo";
        }
        return "bar";
    }

applying the code action on unused "bar", we get:

    public String getLastName() {
        {
            return "foo";
        }
    }

@yaohaizh can you see if getting rid of those surrounding braces is doable? And if you find a solution, you could submit a patch to JDT upstream (https://bugs.eclipse.org/bugs/show_bug.cgi?id=531501)

[fbricon]

@yaohaizh if you can't fix the extra brackets issue now, go ahead and merge this PR so that you can work on testing on #567

[fbricon]

@yaohaizh thanks for the new feature!

Please don't create merge commits. Use either Squash and Merge if you have multiple commits or else Rebase and Merge