refactor: Remove unused Flatpickr code from Edit Task modal

[claremacrae]

Types of changes

Internal changes:

• Refactor (prefix: refactor - non-breaking change which only improves the design or structure of existing code, and making no changes to its external behaviour)

Description

Remove unused Flatpickr-related code from the Edit Task modal...

This code was all remnants of my unreleased earlier ChatGPT-driven attempt at adding a date-picker calendar widget to the Edit Task modal.

That code could have been disabled in #3067, and was finally totally redundant after #3169.

Motivation and Context

Clean up code.

How has this been tested?

• Manual exploratory testing in the demo vault.
• Diffing the new code with the versions prior to adding use of Flatpickr, to confirm I really have removed everything.

Checklist

• My code follows the code style of this project and passes yarn run lint.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• My change has adequate Unit Test coverage.

Terms

• My contribution follows this project's contributing guide
• I agree to follow this project's Code of Conduct

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud