Deprecate `clearProperty(String)`.

This method is no longer needed because RestoreSystemProperties
restores all properties. Now the user can use
`System.clearProperty(String)` in its test.
Deprecate `setProperty(String, String)`.

This method is no longer needed because RestoreSystemProperties
restores all properties. Now the user can use
`System.clearProperty(String, String)` in its test.