Remove `single` and `single_mut` #44990

Job	Run time
build (windows-latest)	8m 21s
check-missing-examples-in-docs	30s
build (ubuntu-latest)	2m 18s
check-doc	3m 36s
build (macos-latest)	1m 51s
ci	28s
miri	6m 28s
run-examples-macos-metal	4m 43s
toml	1m 56s
typos	6s
check-bevy-internal-imports	4s
check-cfg	1m 40s
check-compiles	0s
check-missing-features-in-docs	32s
markdownlint	1m 7s
build-wasm-atomics	1m 28s
build-wasm	44s
msrv	59s
	36m 51s

Provide feedback