Remove forked copy of linked_scroll_controller

[`linked_scroll_controller`](https://pub.dev/packages/linked_scroll_controller) now exists as a separate package. The source-of-truth is still internal, but it internally has been moved out of `dart_src`, and my understanding is that the fork is no longer necessary.

_Originally posted by @jamesderlin in https://github.com/flutter/devtools/pull/1636#issuecomment-602990678_

Start by searching the repository and dart_src for the forked linked_scroll_controller sources and references, then inspect the package dependency configuration. Confirm how the separate linked_scroll_controller package is intended to replace the internal copy. Done means the fork and its references are removed without leaving broken imports or dependency configuration.

由索引模型根据 Issue 内容生成。