refactor: remove unnecessary `Option` in `Async` #604

Job	Run time
windows_gnu (nightly, x86_64-pc-windows-gnu)	2m 15s
msrv (ubuntu-latest, 1.63)	1m 28s
msrv (windows-latest, 1.63)	2m 2s
test (ubuntu-latest, nightly)	1m 32s
test (ubuntu-latest, beta)	1m 5s
test (ubuntu-latest, stable)	56s
test (windows-latest, nightly)	2m 48s
test (windows-latest, beta)	2m 41s
test (windows-latest, stable)	3m 25s
test (macos-latest, nightly)	2m 24s
test (macos-latest, beta)	1m 27s
test (macos-latest, stable)	1m 35s
cross (ubuntu-latest)	2m 18s
cross (macos-latest)	46s
clippy	23s
fmt	9s
security_audit	15s
	27m 29s

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