t-hashmap: stop calling setup() for t_intern() test

t-hashmap: stop calling setup() for t_intern() test

Commit f24a9b78a9 (t-hashmap: mark unused parameters in callback
function, 2024-08-17) noted that the t_intern() does not need its
hashmap parameter, but we have to keep it to conform to the function
pointer interface of setup().

But since the only thing setup() does is create and tear down the
hashmap, we can just skip calling setup() entirely for this case, and
drop the unused parameters. This simplifies the code a bit.

Helped-by: Ghanshyam Thakkar <shyamthakkar001@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>