Remove unused methods `_decompose_into_clifford_*` #20548

Job	Run time
Format check	33s
Isolated pytest Ubuntu	3m 40s
Doc test	2m 20s
Packaging test	2m 10s
Shell check	11s
Type check	1m 20s
Misc check	23s
Notebook formatting	35s
Lint check	2m 22s
Changed files test	1m 57s
Typescript lint check	53s
Bundle file consistency	51s
Pytest Ubuntu (3.14)	3m 31s
Pytest Ubuntu (3.11)	4m 37s
Pytest Ubuntu (3.12)	3m 34s
Pytest Ubuntu (3.13)	3m 37s
Changed Notebooks Isolated Test against Cirq stable (partition-3)	51s
Typescript tests coverage	1m 1s
Changed Notebooks Isolated Test against Cirq stable (partition-1)	51s
Build protos	47s
Coverage check	3m 33s
Changed Notebooks Isolated Test against Cirq stable (partition-0)	51s
Notebook Tests against PR	3m 36s
Pytest MacOS (3.11)	2m 31s
Pytest MacOS (3.12)	2m 43s
Changed Notebooks Isolated Test against Cirq stable (partition-2)	53s
Pytest Windows (3.12)	2m 32s
Pytest MacOS (3.13)	2m 37s
Typescript tests	34s
Check consistency of requirements	24s
Pytest Windows (3.11)	4m 35s
Pytest Windows (3.13)	2m 25s
	1h 3m 18s

Provide feedback