config: remove useless assignment

v1.7.9-8-g270a3443 (config: stop using config_exclusive_filename) replaced
config_exclusive_filename with given_config_file.  In one case this
resulted in a self-assignment, which is reported by clang as a warning.
Remove the useless code.
Signed-off-by: NRene Scharfe <rene.scharfe@lsrfire.ath.cx>
Acked-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/config.c

@@ -397,8 +397,6 @@ int cmd_config(int argc, const char **argv, const char *prefix)
 				xstrdup(prefix_filename(prefix,
 							strlen(prefix),
 							given_config_file));
-		else
-			given_config_file = given_config_file;
 	}
 
 	if (respect_includes == -1)