[FIX] sale_coupon: reset no more eligible applied coupon to Valid state

Before this commit:
When a coupon_program's coupon was giving a reward on a SO and then the
recompute was removing that reward (eg the SO did not met the requirements
anymore) the coupon would not be reset and would be lost.

Now:
If a coupon's reward got removed for some reason, the coupon got reset to a
valid state (`new`) so it can be used again.

Step to reproduce:
  - Generate a coupon from a coupon program that needs requirements (eg $300)
  - Add that coupon to a SO that has a total over $300
  - Coupon is set as `used` and the reward is granted
  - Remove some products to set the SO total below $300
  - Recompute coupon, the reward is correctly removed but the coupon is still
    set as `used` so it won't be usable again and is basically lost.

Closes #3062

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