http: drop unused parameter from start_object_request()

http: drop unused parameter from start_object_request()

We take a "walker" parameter for the request, but don't actually look at
it. This is due to 5424bc557f (http*: add helper methods for fetching
objects (loose), 2009-06-06). Before then, we consulted the "walker"
struct to tell us if we should be verbose, but now those messages are
printed elsewhere.

Let's drop the unused parameter to make -Wunused-parameter happy.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>