Remove unused methods `_decompose_into_clifford_*` #20549

Job	Run time
Misc check	22s
Packaging test	2m 8s
Changed files test	1m 57s
Isolated pytest Ubuntu	7m 26s
Type check	1m 25s
Doc test	2m 26s
Pytest Windows (3.13)	6m 21s
Pytest MacOS (3.13)	2m 33s
Notebook Tests against PR	4m 36s
Pytest Windows (3.12)	5m 49s
Pytest Ubuntu (3.14)	6m 49s
Changed Notebooks Isolated Test against Cirq stable (partition-0)	51s
Lint check	2m 26s
Pytest Ubuntu (3.13)	5m 37s
Format check	40s
Build protos	35s
Pytest Ubuntu (3.11)	4m 52s
Typescript tests coverage	1m 23s
Pytest Ubuntu (3.12)	5m 52s
Shell check	11s
Check consistency of requirements	25s
Typescript lint check	1m 4s
Notebook formatting	31s
Changed Notebooks Isolated Test against Cirq stable (partition-1)	52s
Typescript tests	35s
Changed Notebooks Isolated Test against Cirq stable (partition-2)	54s
Changed Notebooks Isolated Test against Cirq stable (partition-3)	54s
Bundle file consistency	1m 3s
Pytest Windows (3.11)	4m 55s
Pytest MacOS (3.12)	2m 39s
Pytest MacOS (3.11)	2m 33s
Coverage check	9m 56s
	1h 30m 40s

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