config: remove git_config_iter

Since there is no implementation of the function 'git_config_iter',
let's stop exporting it and remove the prototype from config.h.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.h

@@ -165,7 +165,6 @@ extern int git_configset_get_pathname(struct config_set *cs, const char *key, co
 extern int git_config_get_value(const char *key, const char **value);
 extern const struct string_list *git_config_get_value_multi(const char *key);
 extern void git_config_clear(void);
-extern void git_config_iter(config_fn_t fn, void *data);
 extern int git_config_get_string_const(const char *key, const char **dest);
 extern int git_config_get_string(const char *key, char **dest);
 extern int git_config_get_int(const char *key, int *dest);