Remove unused methods `_decompose_into_clifford_*` #20550

Job	Run time
Doc test	2m 35s
Lint check	2m 20s
Format check	33s
Changed files test	1m 54s
Misc check	24s
Type check	1m 15s
Packaging test	1m 57s
Build protos	48s
Bundle file consistency	1m 6s
Notebook formatting	27s
Check consistency of requirements	26s
Pytest Ubuntu (3.14)	6m 6s
Pytest MacOS (3.12)	2m 37s
Pytest MacOS (3.11)	2m 37s
Typescript tests	33s
Coverage check	8m 55s
Pytest MacOS (3.13)	2m 40s
Notebook Tests against PR	4m 57s
Typescript lint check	52s
Shell check	12s
Changed Notebooks Isolated Test against Cirq stable (partition-2)	52s
Pytest Windows (3.11)	5m 8s
Pytest Windows (3.12)	5m 25s
Pytest Ubuntu (3.13)	6m 0s
Pytest Ubuntu (3.11)	4m 52s
Changed Notebooks Isolated Test against Cirq stable (partition-1)	50s
Isolated pytest Ubuntu	7m 27s
Changed Notebooks Isolated Test against Cirq stable (partition-3)	52s
Typescript tests coverage	1m 13s
Pytest Ubuntu (3.12)	5m 28s
Pytest Windows (3.13)	5m 13s
Changed Notebooks Isolated Test against Cirq stable (partition-0)	53s
	1h 27m 27s

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