Remove other custom field dialog and related tests

[laineyhm]

Fixes #1502

Description

This PR removes a custom fields modal created in the lex-modals.utils.ts file and tests that exercised it. This is code that I forgot to clean up in PR #1485.

Type of Change

Code/test cleanup.

Checklist

• I have performed a self-review of my own code
• I have reviewed the title/description of this PR which will be used as the squashed PR commit message
• [N/A] I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works

How to test

• The E2E tests should still all pass

qa.languageforge.org testing

Testers: Check the box and put in a date/time to sign-off/attest the feature works as expected on qa.languageforge.org

• Billy (2022-10-04) – confirmed e2e tests passed during develop merge
• Tester2 (YYYY-MM-DD HH:MM)

[github-actions]

Unit Test Results

368 tests   368 ✔️  8s ⏱️
    1 suites      0 💤
    1 files        0 ❌

Results for commit e379395.

[megahirt]

Great work! LGTM