Remove incorrect declarations in pg_wchar.h.

Oversight in commit 9acae56ce0.

Discussion: https://postgr.es/m/541F240E-94AD-4D65-9794-7D6C316BC3FF@gmail.com

src/include/mb/pg_wchar.h

@@ -676,8 +676,6 @@ extern int	pg_valid_server_encoding(const char *name);
 extern bool is_encoding_supported_by_icu(int encoding);
 extern const char *get_encoding_name_for_icu(int encoding);
 
-extern unsigned char *unicode_to_utf8(char32_t c, unsigned char *utf8string);
-extern char32_t utf8_to_unicode(const unsigned char *c);
 extern bool pg_utf8_islegal(const unsigned char *source, int length);
 extern int	pg_utf_mblen(const unsigned char *s);
 extern int	pg_mule_mblen(const unsigned char *s);