refactor: remove unnecessary `Option` in `Async` #590

Job	Run time
windows_gnu (nightly, x86_64-pc-windows-gnu)	1m 53s
fmt	4s
cross (ubuntu-latest)	2m 35s
cross (macos-latest)	34s
msrv (ubuntu-latest, 1.63)	1m 6s
msrv (windows-latest, 1.63)	1m 43s
security_audit	7s
test (ubuntu-latest, nightly)	1m 14s
test (ubuntu-latest, beta)	1m 12s
test (ubuntu-latest, stable)	57s
test (windows-latest, nightly)	2m 5s
test (windows-latest, beta)	1m 41s
test (windows-latest, stable)	3m 14s
test (macos-latest, nightly)	1m 33s
test (macos-latest, beta)	1m 31s
test (macos-latest, stable)	1m 34s
clippy	35s
	23m 38s

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