Addon-docs: Remove all defaultValue eval-ing

[tmeasday]

Telescoping on #14899

Issue: #14579

What I did

• Simply do not eval and set argType.defaultValue in any case.

What I did not do

• If argType.defaultValue is set, we continue to use it as the initial arg value (if the arg is otherwise unset).

  • This means we do not break backwards compatibility
  • Also it means we don't need to change the implementation of actions' argType enhancer

• I think maybe we can clean up some test cases, @shilman we should probably walk through it together

How to test

• Is this testable with Jest or Chromatic screenshots?

Yes, lots of stories will be affected.

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[shilman]

LGTM!