Remove obsolete ZenModeConfig fields and methods

This is the last step of MODES_UI inlining. The fields previously used for the "global" policy were only used to read from old XML versions and move them to manualRule.zenPolicy.

Bug: 368247671
Flag: EXEMPT Small change with good test coverage
Test: atest ZenModeConfigTest ZenModeDiffTest ZenModeHelperTest
Change-Id: If6f122f861b06968be00bcbcb71d7a10e3dee453