Remove unused json.rs file #5861

Job	Run time
Build docs	41s
Test (macOS)	2m 22s
Test (nightly)	1m 53s
Test (stable)	2m 44s
Test (beta)	3m 10s
Test (Windows)	5m 7s
Test (native-tls)	2m 46s
Test (no default features)	1m 7s
Test (no cache)	2m 35s
Examples	3m 15s
Test (no gateway)	1m 56s
Test (unstable Discord API features)	2m 48s
Check minimal versions	29s
Test (zlib compression)	2m 41s
Test (zstd compression)	3m 7s
Test (zlib and zstd compression)	2m 49s
MSRV	0s
Test (chrono, chrono)	1m 13s
Test (unstable API + typesize, default unstable typesize, true)	1m 22s
Test (builder without model, builder, true)	51s
Test (unstable Discord API (no default features), unstable, true)	1m 6s
	44m 2s

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