coverage: delete old string type coverageDecorator setting handling

[hyangah]

Since 2018 June release go.coverageDecorator has been object type (which allows highlight, gutter customization). The extension has code that accepts the old string type setting and updates users' settings.json with the new object type. It's time to clean up the code.

cc @pjweinbgo

[gopherbot]

Change https://golang.org/cl/251337 mentions this issue: src/goCover: remove obsolete go.coverageDecorator support