Remove `EventType` enum #672

Job	Run time
Test (macOS)	3m 47s
Examples	3m 51s
Build docs	31s
Check minimal versions	53s
MSRV	0s
Test (nightly)	4m 13s
Test (stable)	4m 37s
Test (beta)	5m 10s
Test (Windows)	7m 18s
Test (native-tls)	6m 48s
Test (no default features)	55s
Test (no cache)	5m 36s
Test (no gateway)	3m 13s
Test (unstable Discord API features)	1m 44s
Test (simd-json)	6m 57s
Test (chrono, chrono)	59s
Test (builder without model, builder, true)	28s
Test (unstable Discord API (no default features), unstable_discord_api, true)	41s
	57m 41s

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