Remove 0xf6 empty memo convention. #3354

Job	Run time
Book tests	13s
Build target wasm32-wasi	37s
Bitrot check	1m 6s
Latest build on ubuntu-latest	1m 2s
Build target thumbv7em-none-eabihf	32s
Code coverage	8m 3s
Latest build on windows-latest	2m 52s
Intra-doc links	1m 30s
Latest build on macOS-latest	50s
Rustfmt	12s
Test on ubuntu-latest	8m 56s
Test on windows-latest	10m 35s
Test on macOS-latest	8m 27s
	44m 55s

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