impl(spanner): remove deprecated Admin classes (#15862) #1018

Job	Run time
debug_job	3s
Save PR ref	4s
Windows-Bazel / bazel + msvc-2022 + opt + Core	29s
macOS-Bazel	0s
Windows-CMake	0s
macOS-CMake	0s
Notify-Google-Chat	2s
	38s

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