cut legacy backcompat alias folly::io::StreamCodec

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

Reviewed By: skrueger

Differential Revision: D68536731

fbshipit-source-id: 661f74c0f611f7f0b28e3bdaea1f868a6ecd2ceb

Author: yfeldblum

folly/compression/Compression.h

@@ -546,5 +546,4 @@ using folly::compression::CodecType;
 using folly::compression::getAutoUncompressionCodec;
 using folly::compression::getCodec;
 using folly::compression::hasCodec;
-using folly::compression::StreamCodec;
 } // namespace folly::io