fix: simplify Enter key handling and remove unused ref

[SuzukiKatsuma]

Checklist:

Important

Please review the checklist below before submitting your pull request.

• Please open an issue before creating a PR or link to an existing issue
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

Description

Fix #9295

There are times when input is mistakenly sent by pressing enter after completing IME input. When inserting a line break with shift + enter, mistaken sending occurs if you:

1. press down shift and enter
2. release shift
3. release enter

The issue described above has been resolved.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update, included: Dify Document
• Improvement, including but not limited to code refactoring, performance optimization, and UI/UX improvement
• Dependency upgrade

Testing Instructions

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Create a sample chatbot
• Use IME in the chat-input section within the preview
• Confirm that after inserting a line break with Shift + Enter, releasing Shift first and then releasing Enter with a slight delay does not send the chat message.