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.

Written by the indexing model from the issue text.