Remove MantidKernel/System.h from unit tests

[peterfpeterson]

Importing less means the pre-processor and compiler have less to do. Also remove 3 unit tests that are completely empty and are simply from class_maker.py (Framework/API/test/IEventListTest.h, Framework/Kernel/test/IPropertySettingsTest.h, and Framework/Kernel/test/ISaveableTest.h).

Refs #38457

To test:

This is a simple refactor so the automated tests should continue to pass.

This does not require release notes because it is a refactor.

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