Add delete email template for admin and unit tests

[dsmabulage]

Purpose

The purpose of this PR is to fix #41

Goals

• Implement delete email template functionality for admins

Approach

• create a separate method for deleting email templates in the email template service.
• write a unit test for functionality.

Screenshots

Checklist

• This PR doesn't commit any keys, passwords, tokens, usernames, or other secrets.
• I have read and understood the development best practices guidelines ( http://bit.ly/sef-best-practices )
• 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 have made corresponding changes to the documentation

Related PRs

Test environment

Learning

[anjula-sack]

Thank you! @dsmabulage