revert adding :transcode mode for startproc

src/codec.jl

@@ -76,10 +76,9 @@ the stream will become the close mode for safety.
 
 The `startproc(codec::C, mode::Symbol, error::Error)::Symbol` method takes
 `codec`, `mode` and `error`, and returns a status code. This is called just
-before the stream starts reading or writing data. `mode` is either `:read`,
-`:write`, or `:transcode` and then the stream starts reading or
-writing, or one of the `transcode` functions is used. 
-The return code must be `:ok` if `codec` is ready for data. Otherwise,
+before the stream starts reading or writing data. `mode` is either `:read` or
+`:write` and then the stream starts reading or writing, respectively.  The
+return code must be `:ok` if `codec` is ready to read or write data.  Otherwise,
 it must be `:error` and the `error` argument must be set to an exception object.
 
 ### `process`

src/transcode.jl

@@ -144,8 +144,7 @@ function unsafe_transcode!(
 )
     error = Error()
     # Note: pledged_input_size is currently experimental
-    # :transcode mode enables some codecs to error if they are used in a TranscodingStream.
-    code = startproc2(codec, :transcode, error; pledged_input_size=buffersize(input))
+    code = startproc2(codec, :write, error; pledged_input_size=buffersize(input))
     if code === :error
         @goto error
     end
@@ -168,7 +167,7 @@ function unsafe_transcode!(
         @goto error
     elseif code === :end
         if buffersize(input) > 0
-            if startproc2(codec, :transcode, error) === :error
+            if startproc2(codec, :write, error) === :error
                 @goto error
             end
             n = minoutsize(codec, buffermem(input))