Remove usages of bugfix flag cleanup_accessibility_warning_dialog.

Flag has been in Trunkfood for over a month.

Includes one mechanical refactor to move a method from a now-removed
file into a private method in the class that uses it: createDisableDialog

(This flag guarded a preliminary cleanup and minor bug fix for
b/303511250, but the actual feature for b/303511250 is tracked in a
different flag 'skip_accessibility_warning_dialog_for_trusted_services')

Bug: 303511250
Test: existing tests in Presubmit (no change to behavior)
Change-Id: Iccf439adb2806e988310f503fb3891a6bd022054