Update vendored libs

vendor/github.com/DataDog/zstd/README.md

@@ -6,8 +6,8 @@
 
 [C Zstd Homepage](https://github.com/facebook/zstd)
 
-The current headers and C files are from *v1.5.6* (Commit
-[794ea1b](https://github.com/facebook/zstd/releases/tag/v1.5.6)).
+The current headers and C files are from *v1.4.4* (Commit
+[10f0e699](https://github.com/facebook/zstd/releases/tag/v1.4.4)).
 
 ## Usage
 
@@ -19,21 +19,6 @@ There are two main APIs:
 The compress/decompress APIs mirror that of lz4, while the streaming API was
 designed to be a drop-in replacement for zlib.
 
-### Building against an external libzstd
-
-By default, zstd source code is vendored in this repository and the binding will be built with
-the vendored source code bundled.
-
-If you want to build this binding against an external static or shared libzstd library, you can
-use the `external_libzstd` build tag. This will look for the libzstd pkg-config file and extract
-build and linking parameters from that pkg-config file.
-
-Note that it requires at least libzstd 1.4.0.
-
-```bash
-go build -tags external_libzstd
-```
-
 ### Simple `Compress/Decompress`
 
 
@@ -75,9 +60,6 @@ NewWriterLevelDict(w io.Writer, level int, dict []byte) *Writer
 // Write compresses the input data and write it to the underlying writer
 (w *Writer) Write(p []byte) (int, error)
 
-// Flush writes any unwritten data to the underlying writer
-(w *Writer) Flush() error
-
 // Close flushes the buffer and frees C zstd objects
 (w *Writer) Close() error
 ```