src: simplify handling of kNoAuthTagLength #2122

Job	Run time
Build slim tarball	1m 34s
x86_64-darwin: with shared libraries	47m 5s
x86_64-linux: with shared libraries	27m 10s
aarch64-darwin: with shared libraries	26m 48s
aarch64-linux: with shared libraries	22m 29s
	2h 5m 6s

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