[mux] Remove degenerate subgroup support. #568

Job	Run time
Call PR testing on schedule / run_host_x86_64	28m 30s
Call PR testing on schedule / run_clang_format	10s
Call PR testing on schedule / run_clang_tidy_changes	21m 55s
Call PR testing on schedule / run_riscv_m1	23m 48s
Call PR testing on schedule / run_ubuntu_gcc_x86_64_riscv_fp16_cl3_0_unitcl_vecz	51m 36s
Call PR testing on schedule / run_windows_msvc_x86_64_llvm_latest_cl3_0_offline	43m 22s
Call PR testing on schedule / run-ubuntu-clang-x86-llvm-latest-cl3-0-offline	29m 29s
Call PR testing on schedule / run-ubuntu-gcc-x86_64-riscv-fp16-cl3-0	46m 41s
Call PR testing on schedule / run-ubuntu-gcc-aarch64-llvm-latest-cl3-0-fp16	8m 35s
Call PR testing on schedule / run-ubuntu-gcc-x86_64-clik-refsi	1m 19s
Call PR testing on schedule / run-ubuntu-gcc-x86_64-refsi-g1-wi-cl3-0	0s
Call PR testing on schedule / run-ubuntu-gcc-x86-llvm-latest-x86_64-images-cl3-0-release	31m 53s
	4h 47m 18s

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