remote.c: drop extraneous local variable from migrate_file

remote.c: drop extraneous local variable from migrate_file

It's an anti-pattern to assign the result of git_path to a
variable, since other calls may reuse our buffer. In this
case, we feed the result to unlink_or_warn immediately
afterwards, so it's OK. However, it's nice to avoid
assignment entirely, which makes it more obvious that
there's no bug.

We can just pass the result directly to unlink_or_warn,
which is a known-simple function. As a bonus, the code flow
is a little more obvious, as we eliminate an extra
conditional (a reader does not have to wonder any more
"under which circumstances is 'path' set?").

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