Remove unused prototype _PyPegen_set_source_in_metadata (#138810)

Remove unused prototype

Author: wiomoc

Parser/pegen.h

@@ -162,7 +162,6 @@ int _PyPegen_fill_token(Parser *p);
 expr_ty _PyPegen_name_token(Parser *p);
 expr_ty _PyPegen_number_token(Parser *p);
 void *_PyPegen_string_token(Parser *p);
-PyObject *_PyPegen_set_source_in_metadata(Parser *p, Token *t);
 Py_ssize_t _PyPegen_byte_offset_to_character_offset_line(PyObject *line, Py_ssize_t col_offset, Py_ssize_t end_col_offset);
 Py_ssize_t _PyPegen_byte_offset_to_character_offset(PyObject *line, Py_ssize_t col_offset);
 Py_ssize_t _PyPegen_byte_offset_to_character_offset_raw(const char*, Py_ssize_t col_offset);