Remove double mention of `poise` #877

Job	Run time
Test (macOS)	2m 12s
Check minimal versions	32s
MSRV	35s
Test (nightly)	2m 44s
Test (stable)	2m 55s
Build docs	24s
Examples	2m 48s
Test (beta)	2m 47s
Test (Windows)	4m 32s
Test (native-tls)	2m 49s
Test (no default features)	36s
Test (no cache)	2m 38s
Test (no gateway)	1m 44s
Test (unstable Discord API features)	2m 57s
Test (simd-json)	2m 54s
Test (chrono, chrono)	33s
Test (unstable API + typesize, default unstable_discord_api typesize, true)	1m 10s
Test (builder without model, builder, true)	21s
Test (unstable Discord API (no default features), unstable_discord_api, true)	22s
	35m 33s

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