src: simplify handling of kNoAuthTagLength #1875

Job	Run time
Build slim tarball	1m 32s
x86_64-darwin: with shared libraries	44m 33s
aarch64-darwin: with shared libraries	27m 42s
x86_64-linux: with shared libraries	29m 48s
aarch64-linux: with shared libraries	25m 5s
	2h 8m 40s

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