remove Identity codec completely (#41)

Author: bicycle1885

src/TranscodingStreams.jl

@@ -16,7 +16,6 @@ include("codec.jl")
 include("state.jl")
 include("stream.jl")
 include("io.jl")
-include("identity.jl")
 include("noop.jl")
 include("transcode.jl")
 include("testtools.jl")

src/identity.jl

@@ -129,7 +129,6 @@ end
 include("codecnoop.jl")
 include("codecinvalid.jl")
 include("codecquadruple.jl")
-include("codecidentity.jl")  # deprecated codec
 
 # Test third-party codec packages.
 for pkg in ["CodecZlib", "CodecBzip2", "CodecXz", "CodecZstd", "CodecBase"]