Remove `InternalEventHandler` (#2952) #948

Job	Run time
Examples	2m 51s
Build docs	42s
Check minimal versions	31s
Test (macOS)	1m 28s
MSRV	0s
Test (nightly)	1m 45s
Test (stable)	2m 35s
Test (beta)	2m 38s
Test (Windows)	4m 21s
Test (native-tls)	2m 18s
Test (no default features)	58s
Test (no cache)	2m 31s
Test (no gateway)	1m 44s
Test (unstable Discord API features)	2m 42s
Test (chrono, chrono)	50s
Test (unstable API + typesize, default unstable typesize, true)	1m 7s
Test (builder without model, builder, true)	31s
Test (unstable Discord API (no default features), unstable, true)	32s
	30m 4s

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