mailmap: remove redundant check for freeing memory

The condition as it is written in that line has already been checked
in the beginning of the function, which was introduced in
8503ee4 (2007-05-01, Fix read_mailmap to handle a caller uninterested
in repo abbreviation)

Helped-by: Jeff King <peff@peff.net>
Helped-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mailmap.c

@@ -153,8 +153,7 @@ static void read_mailmap_line(struct string_list *map, char *buffer,
 		if (!strncmp(buffer, abbrev, abblen)) {
 			char *cp;
 
-			if (repo_abbrev)
-				free(*repo_abbrev);
+			free(*repo_abbrev);
 			*repo_abbrev = xmalloc(len);
 
 			for (cp = buffer + abblen; isspace(*cp); cp++)