src: remove SetEncoding from StringEncoder

PR-URL: https://github.com/nodejs/node/pull/53441
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Daniel Lemire <daniel@lemire.me>

src/string_decoder.h

@@ -10,7 +10,6 @@ namespace node {
 class StringDecoder {
  public:
   StringDecoder() { state_[kEncodingField] = BUFFER; }
-  inline void SetEncoding(enum encoding encoding);
   inline enum encoding Encoding() const;
 
   inline char* IncompleteCharacterBuffer();