Remove rustflags #5370

Job	Run time
Build docs	26s
Examples	2m 1s
Test (macOS)	1m 4s
Check minimal versions	28s
MSRV	0s
Test (nightly)	2m 12s
Test (stable)	2m 13s
Test (beta)	2m 3s
Test (Windows)	4m 13s
Test (native-tls)	2m 6s
Test (no default features)	38s
Test (no cache)	2m 26s
Test (no gateway)	1m 26s
Test (unstable Discord API features)	2m 19s
Test (chrono, chrono)	34s
Test (unstable API + typesize, default unstable typesize, true)	53s
Test (builder without model, builder, true)	17s
Test (unstable Discord API (no default features), unstable, true)	31s
	25m 50s

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