Remove function addPackageToPackageJson

[kauppfbi]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Performance issue
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request
[x] Refactoring Task

Current behavior

In the current implementation, the ng add schematic manually writes into the package.json in order to add the package as devDependency. Meanwhile, the angular devkit schematics package provides a easy way to do so, see https://github.com/angular/angular-cli/pull/15815

Expected behavior

Use the angular way and get rid of the unused code.

[JohannesHoppe]

FYI, this was added to npx-deploy-starter here: https://github.com/angular-schule/ngx-deploy-starter/commit/d5f61a263e900d7bb31b7860c6982d89f7ea3cc8