Fix unused warnings when builder feature is disabled #960

Job	Run time
Examples	2m 2s
Check minimal versions	29s
Test (macOS)	1m 7s
Build docs	20s
MSRV	0s
Test (nightly)	1m 16s
Test (stable)	2m 30s
Test (beta)	2m 13s
Test (Windows)	4m 25s
Test (native-tls)	2m 19s
Test (no default features)	37s
Test (no cache)	2m 15s
Test (no gateway)	1m 32s
Test (unstable Discord API features)	2m 29s
Test (chrono, chrono)	37s
Test (unstable API + typesize, default unstable typesize, true)	48s
Test (builder without model, builder, true)	30s
Test (unstable Discord API (no default features), unstable, true)	23s
	25m 52s

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