pretty: remove reencode_commit_message()

This function has only two callsites, and is a thin wrapper whose
usefulness is dubious.  When the caller needs to learn the log
output encoding, it should be able to do so by directly calling
get_log_output_encoding() and calling the underlying
logmsg_reencode() with it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.h

@@ -99,8 +99,6 @@ extern int has_non_ascii(const char *text);
 struct rev_info; /* in revision.h, it circularly uses enum cmit_fmt */
 extern char *logmsg_reencode(const struct commit *commit,
 			     const char *output_encoding);
-extern char *reencode_commit_message(const struct commit *commit,
-				     const char **encoding_p);
 extern void get_commit_format(const char *arg, struct rev_info *);
 extern const char *format_subject(struct strbuf *sb, const char *msg,
 				  const char *line_separator);