Backport 2.2 - Don't set a source model on the attribute when it's no…

[hostep]

…t needed. This avoids accidentally persisting the source model to the database when using multiselect attributes.

(cherry picked from commit 477ca51)

Description

This is a backport of #18244 for Magento 2.2, see original PR for full description.

Fixed Issues (if relevant)

1. Updating attribute option data through API will set unwanted source_model on the attribute #13156: Updating attribute option data through API will set unwanted source_model on the attribute

Manual testing scenarios

See original PR

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)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @aleron75, thank you for the review.
ENGCOM-3128 has been created to process this Pull Request

[magento-engcom-team]

Hi @hostep. Thank you for your contribution.
We will aim to release these changes as part of 2.2.8.
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.