fetch: stop passing around unused worktrees variable

fetch: stop passing around unused worktrees variable

In 12d47e3b1f (fetch: use new branch_checked_out() and add tests,
2022-06-14), fetch's update_local_ref() function stopped using its
"worktrees" parameter. It doesn't need it, since the
branch_checked_out() function examines the global worktrees under the
hood.

So we can not only drop the unused parameter from that function, but
also from its entire call chain. And as we do so all the way up to
do_fetch(), we can see that nobody uses it at all, and we can drop the
local variable there entirely.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>