Update eslintrc no-param-reassign to allow props reassignment

What does this MR do?

(Issue identified in this thread https://gitlab.slack.com/archives/C0GQHHPGW/p1535108676000100)

• Update .eslintrc.yml to allow prop reassignment for the rule no-param-reassign
• Removes unused eslint-disable that this change caused.

EE Port

Please note the EE port of this MR (gitlab-ee!7001).

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides