refactor: remove unnecessary `Option` in `Async` #586

Job	Run time
clippy	28s
msrv (ubuntu-latest, 1.63)	1m 20s
msrv (windows-latest, 1.63)	1m 38s
fmt	12s
cross (ubuntu-latest)	5m 27s
cross (macos-latest)	2m 30s
security_audit	10s
test (ubuntu-latest, nightly)	1m 22s
test (ubuntu-latest, beta)	3m 11s
test (ubuntu-latest, stable)	1m 10s
test (windows-latest, nightly)	1m 51s
test (windows-latest, beta)	1m 56s
test (windows-latest, stable)	2m 47s
test (macos-latest, nightly)	1m 35s
test (macos-latest, beta)	1m 26s
test (macos-latest, stable)	1m 17s
windows_gnu (nightly, x86_64-pc-windows-gnu)	2m 7s
	30m 27s

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