build.gradle: remove `idea` plugin
build.gradle: remove `idea` plugin

The `idea` plugin implements the `idea` task which helps developers
generate IDEA configuration files for use in the IDE.

However, this method of using IntelliJ IDEA for development in our code
base is not supported by us. Instead, developers should import the
project into IntelliJ IDEA using its "import Gradle project"
functionality.

To avoid misleading people, let's remove this plugin.

se-edu/addressbook-level3,damithc/addressbook-level4,damithc/addressbook-level4,damithc/addressbook-level4