Simplify the DimensionContentCollection class

[niklasnatter]

After #174 is merged, the DimensionContentCollection class will be one of the main classes of the bundle. Because of this, I think it is important to keep the class simple and consistent. Therefore I would propose the following changes:

• a) The DimensionContentCollection should not extend \Traversable or \Countable. This makes the implementation of the class more complicated without improving DX. As a developer, I would expect a getDimensionContents method that returns an array of DimensionContents.

• b) Remove the getUnlocalizedDimensionContent method and getLocalizedDimensionContent method. To keep things flexible, all services should use the getDimensionContent method with a respective dimensionAttributes parameter (see Adjust interface of services to accept locale instead of dimensionAttributes array #84 (comment)). By removing the methods, we force our services to consistently handle the dimensionAttributes parameter. (fixed in Pass DimensionContentCollection to DataMapper services #182)

After these changes, the DimensionContentCollectionInterface should look something like this:

interface DimensionContentCollectionInterface
{
    /**
     * @return DimensionContentInterface[]
     */
    public function getDimensionContents(): array;

    /**
     * @param mixed[] $dimensionAttributes
     */
    public function getDimensionContent(array $dimensionAttributes): ?DimensionContentInterface;

    /**
     * @return class-string<DimensionContentInterface>
     */
    public function getDimensionContentClass(): string;

    /**
     * @return mixed[]
     */
    public function getDimensionAttributes(): array;
}