DevTools: Remove unused code after service/shared worker inspection migration to main thread.

BUG=404355
R=yurys, pfeldman, horo

Review URL: https://codereview.chromium.org/670433002

git-svn-id: svn://svn.chromium.org/blink/trunk@184056 bbb929c8-8fbe-4397-9dbb-9b2b20218538