Fix build without zlib compression enabled

Summary: The namespace alias was trying to alias a namespace that doesn't exist unless Folly is being compiled with zlib support.

Reviewed By: terrelln

Differential Revision: D5810929

fbshipit-source-id: c659d8f775fc9b99e57cc95f5914418507e546fb

folly/io/Compression.cpp

@@ -62,8 +62,6 @@
 using folly::io::compression::detail::dataStartsWithLE;
 using folly::io::compression::detail::prefixToStringLE;
 
-namespace zlib = folly::io::zlib;
-
 namespace folly {
 namespace io {