walker: drop fields of `struct walker` which are always 1

walker: drop fields of `struct walker` which are always 1

After the previous commit, both users of `struct walker` set `get_tree`,
`get_history` and `get_all` to 1. Drop those fields and simplify the
walker implementation accordingly.

Let's hope that any out-of-tree users will not mind this change. They
should notice that the compilation fails as they try to set these
fields. (If they do not set them, note that `get_http_walker()` leaves
them undefined, so the behavior will have been undefined all the time.)

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>