Remove unnecessary `cfg` directive #155

Job	Run time
rustfmt	7s
macOS	29s
Windows	2m 27s
Linux (default features)	28s
Linux (all features)	28s
Linux (minimal)	17s
Linux (!std + decoder)	24s
Linux (encoder)	16s
Linux (!std + full-decoder + encoder + formatter)	16s
	5m 12s

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