(maint) Removes irrelevant macOS tests

Starting with macOS 10.14 Mojave, Apple require full disk access
(FDA) to make changes to certain system functionality, including
user ID (UID) and the `/home` directory. Granting FDA means either
manual user intervention or creating a Privacy Preferences
Policy Control Payload (PPPCP) profile through mobile device
management (MDM).

This means that these two tests (`should_modify_home.rb` and
`should_modify_uid.rb`) don't work on any version of macOS that
Puppet supports (10.14-11), so we no longer need them.

Author: mhashizume