Canvas AI: Remove field_name from agents

Overview

CanvasBuilder::render() logic was simplified in #3555464: CanvasBuilder::render() logic around specific tool response is unclear. This issue is a follow-up to test and remove unused field_name from modules/canvas_ai/src/Plugin/AiFunctionCall/EditFieldContent.php and modules/canvas_ai/src/Plugin/AiFunctionCall/CreateFieldContent.php
See: https://git.drupalcode.org/project/canvas/-/merge_requests/286#note_620997

Proposed resolution

User interface changes