Remove support for COMBINED_SIGNAL_ICONS

This change removes all of the branches associated with the combined
signal iconography implemented for the provider model behavior for the
following reasons:

1. This flag has been false for a while now, ever since the split out into
   two parts (provider model behavior + provider model setting).
2. The original implementation of this behavior sits at the wrong level
   of abstraction for maintainability and understandability, due to the
   constraints on building that feature out.
3. In order to put the domain logic for this iconography into a better
   place, it makes sense to remove these unused branches now, and
   re-implement in the future if we need to bring it back

Test: atest SystemUITests
Bug: 238425913
Change-Id: I1bef3f309058136f190b1d4494a1a53022786a4f