Fix function signatures for `ChannelId::delete_reactions` and `Messag… #4292

Job	Run time
Examples	2m 33s
Test (macOS)	3m 37s
Check minimal versions	1m 8s
Build docs	3m 0s
MSRV	3m 3s
Test (nightly)	3m 39s
Test (stable)	1m 23s
Test (beta)	1m 25s
Test (Windows)	3m 48s
Test (native-tls)	4m 47s
Test (no default features)	44s
Test (no cache)	5m 47s
Test (no gateway)	3m 44s
Test (time)	1m 9s
Test (unstable Discord API features)	1m 38s
Test (simd-json)	1m 22s
Test (chrono, chrono)	1m 12s
Test (builder without model, builder, true)	31s
Test (unstable Discord API (no default features), unstable_discord_api, true)	34s
	45m 4s

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