cut legacy backcompat alias folly::io::Codec

Summary: All uses have been migrated so the unused alias may now be removed.

Reviewed By: DenisYaroshevskiy

Differential Revision: D68639722

fbshipit-source-id: bd02513fb74541c03c1eaaabddb7a4460ebc75e0

Author: yfeldblum

folly/compression/Compression.h

@@ -541,6 +541,5 @@ bool hasStreamCodec(CodecType type);
 } // namespace folly
 
 namespace folly::io {
-using folly::compression::Codec;
 using folly::compression::CodecType;
 } // namespace folly::io