remove mux.md #230

Job	Run time
Bitrot check	1m 38s
Build target wasm32-unknown-unknown	2m 43s
Intra-doc links	2m 12s
Build target wasm32-wasi	2m 33s
Book tests	2m 20s
Rustfmt	11s
Test on i686-unknown-linux-gnu	13m 59s
Test on ubuntu-latest	8m 5s
Test on windows-latest	10m 14s
Test on macOS-latest	9m 3s
Test on i686-unknown-linux-gnu with beta features	13m 40s
Test on ubuntu-latest with beta features	8m 28s
Test on i686-unknown-linux-gnu with nightly features	16m 27s
Test on windows-latest with beta features	10m 47s
Test on macOS-latest with beta features	8m 11s
Test on ubuntu-latest with nightly features	8m 57s
Test on windows-latest with nightly features	11m 29s
Test on macOS-latest with nightly features	11m 32s
	2h 22m 29s

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