Remove direct use of object manager

[AnshuMishra17]

Description

Remove the direct use of object manager and loaded the dependency via constructor dependency injection.

Manual testing scenarios

1. Unsubscribe the newsletter subscribers using the unsubscription mass action in admin.
2. Deleted the newsletter subscribers using the delete mass action in admin.

[magento-engcom-team]

Hi @AnshuMishra17. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
• @magento-engcom-team give me {$VERSION} instance - deploy vanilla Magento instance

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[magento-engcom-team]

Hi @ihor-sviziev, thank you for the review.
ENGCOM-2379 has been created to process this Pull Request

[magento-engcom-team]

Hi @AnshuMishra17. Thank you for your contribution.
We will aim to release these changes as part of 2.2.6.
Please check the release notes for final confirmation.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.