🚀 Feature: Also remove existing "mocha" package.json field

[JoshuaKGoldberg]

Bug Report Checklist

• I have pulled the latest main branch of the repository.
• I have searched for related issues and found none that matched my issue.

Overview

#1372 tracks deleting any existing Mocha options file in a migration... but Mocha can alternately be configured with a "mocha" key in the package.json of a project. Let's have migration also delete that "mocha" in the package.json if it exists.

Additional Info

The logic to remove existing unnecessary fields in a package.json is around here:

create-typescript-app/src/steps/writing/creation/writePackageJson.ts

Lines 55 to 59 in c5cac23

	// Remove fields we know we don't want, such as old or redundant configs
	eslintConfig: undefined,
	husky: undefined,
	prettierConfig: undefined,
	types: undefined,

Similar issue for Jest: #1372