Remove incorrect is_multi_pledgor fields from pledge_pledgors.items

[IlyaKislitsin]

Description

Remove incorrect is_multi_pledgor fields from pledge_pledgors.items

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I wrote unit tests for my code (if tests is required for my changes)
• I have made changes in CHANGELOG.md file

Changelog

Removed

• Field with path pledge_pledgors.items[].pledgors[].is_multi_pledgor
• Field with path pledge_pledgors.items[].pledgees[].is_multi_pledgor