src: remove unused private field in `SQLTagStore` #1555

Job	Run time
Build slim tarball	1m 34s
x86_64-linux: with shared libraries	29m 33s
aarch64-linux: with shared libraries	25m 32s
aarch64-darwin: with shared libraries	23m 7s
x86_64-darwin: with shared libraries	46m 31s
	2h 6m 17s

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