Remove broken `rest-grpc-multiplex` example (#3105) #5169

Job	Run time
cargo-public-api-crates (axum)	32s
cargo-hack	5m 36s
check-docs	22s
check	59s
cargo-public-api-crates (axum-core)	21s
dependencies-are-sorted	24s
cargo-public-api-crates (axum-extra)	28s
cargo-deny check (advisories)	22s
Spell Check with Typos	5s
cargo-public-api-crates (axum-macros)	25s
cargo-deny check (bans licenses sources)	24s
test-msrv	2m 7s
test-docs	1m 35s
test-nightly	1m 35s
armv5te-unknown-linux-musleabi	38s
test-versions (stable)	2m 47s
wasm32-unknown-unknown	22s
test-versions (beta)	2m 19s
	21m 21s

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