Fix variant selection on product detail page

[shochdoerfer]

Fixes #22

The AddToCartFormComponent class re-uses the ComponentWithFormTrait even though it extends the Sylius base component which already uses it. This creates two separate copies of the private properties and private methods which seems to cause an issue. Removing the trait fixes the issue, the variant selector is now properly refreshing the summary component again.

[crydotsnake]

CC @GSadee / @TheMilek

[jkindly]

The trait is already provided by the parent BaseAddToCartFormComponent, so re-declaring it created a duplicate set of the trait's private properties ($form, $formView, …) in the child scope, desynchronizing the form state and preventing the price from refreshing on variant change. I verified that the child inherits the trait (and its abstract instantiateForm() contract) entirely through the parent - no method here relies on a local copy of the trait - so this is a safe, behavior-preserving change.