ember/no-assignment-of-untracked-properties-used-in-tracking-contexts should not be fixable

Since there are multiple ways to fix this error, the autofixer should not arbitrarily choose one. I had to disable this rule in the project I'm working on because it's adding calls to `set()` when what I really needed to do was make the property `@tracked`.

(As a side note, it also added an import from `@ember/object` that conflicted with an existing import, causing a different lint error. There may be an unrelated problem with that rule.)

cc @bmish