gh-86085: Remove _PyCodec_Forget() declaration (#106377)

The code was already removed by:
commit c9f696c.

Author: vstinner

Include/internal/pycore_codecs.h

@@ -6,8 +6,6 @@ extern "C" {
 
 extern PyObject* _PyCodec_Lookup(const char *encoding);
 
-extern int _PyCodec_Forget(const char *encoding);
-
 /* Text codec specific encoding and decoding API.
 
    Checks the encoding against a list of codecs which do not