Remove legacy catalog_category_product_index query

[mohaelmrabet]

Description (*)

This PR removes the problematic getProductsCountQuery() method that was querying the catalog_category_product_index table, which is always empty in modern Magento installations using store-segmented indexing.

Note: No changes were made to the constructor - backward compatibility is fully maintained through existing nullable parameters and ObjectManager fallbacks.

Fixed Issues (if relevant)

1. Fixes Incorrect index table used in getProductsCountQuery() (fallback masks issue) #40320

Manual testing scenarios (*)

1. Load a category collection with product counts enabled:

$collection = $objectManager->create(\Magento\Catalog\Model\ResourceModel\Category\Collection::class);
$collection->setLoadProductCount(true)->load();

2. Verify in SQL logs that queries use catalog_category_product table and NOT catalog_category_product_index
   Confirm that product counts are accurate for categories across different stores
3. Test with both small category sets (<400) and large category sets (>400) to ensure both code paths work correctly
   Verify product counts respect website/store filtering
4. Test multi-website setup: Create products assigned to different websites and verify each website's category shows correct product count

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• README.md files for modified modules are updated and included in the pull request if any README.md predefined sections require an update
• All automated tests passed successfully (all builds are green)

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[mohaelmrabet]

@magento run all tests

[mohaelmrabet]

@magento run Static Tests

[mohaelmrabet]

@magento run Unit Tests

[mohaelmrabet]

@magento run all tests

[mohaelmrabet]

@magento run Functional Tests CE