refactor!: remove files_changed module

BREAKING CHANGE: Removed `files_changed.py`. Use the `files_changed`
parameter or `taskgraph.util.vcs.Repository.get_changed_files` instead.
Repos hosted on `hg.mozilla.org` that were relying this module will need
to re-implement
`files_changed._get_changed_files_json_automationrelevance` themselves.

Author: ahal

docs/reference/source/taskgraph.rst

@@ -48,14 +48,6 @@ taskgraph.docker module
    :undoc-members:
    :show-inheritance:
 
-taskgraph.files\_changed module
--------------------------------
-
-.. automodule:: taskgraph.files_changed
-   :members:
-   :undoc-members:
-   :show-inheritance:
-
 taskgraph.filter\_tasks module
 ------------------------------