[mux] Remove degenerate subgroup support.

[hvdijk]

Overview

[mux] Remove degenerate subgroup support.

Reason for change

Older versions of oneAPI Construction Kit used an implementation of subgroups where the subgroup size was always equal to the workgroup size. This implementation is no longer used by any targets, but the code to support it was still in place. This commit removes it.

Description of change

Describe the intended behaviour your changes are meant to introduce to the
project and explain how they resolve the problem stated above. Detail any
relevant changes that may affect other users of the project, such as
compilation options, runtime flags, expected inputs and outputs, API entry
points, etc.

If you have added new testing, provide details on what tests you have added
and what the purpose of them is.

Anything else we should know?

If there's any other relevant information we should know that may help us in
understanding and verifying your patch, please include it here.

Checklist

• Read and follow the project Code of Conduct.
• Make sure the project builds successfully with your changes.
• Run relevant testing locally to avoid regressions.
• Run clang-format-19 on all modified code.