string-list: remove unused "insert_at" parameter from add_entry

string-list: remove unused "insert_at" parameter from add_entry

In "add_entry", we accept "insert_at" parameter which must be either -1
(auto) or between 0 and `list->nr` inclusive. Any other value is
invalid. When caller specify any invalid "insert_at" value, we won't
check the range and move the element, which would definitely cause the
trouble.

However, we only use "add_entry" in "string_list_insert" function and we
always pass the "-1" for "insert_at" parameter. So, we never use this
parameter to insert element in a user specified position.

And we should know why there is such code path in the first place. We
used to have another function "string_list_insert_at_index()", which
uses the extra "insert_at" parameter. And in f8c4ab611a (string_list:
remove string_list_insert_at_index() from its API, 2014-11-24), we
remove this function but we don't clean all the code path.

Let's simply delete this parameter as we'd better use "strmap" for such
functionality.

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