Revert "Save cookies back to jar when fetching clone.bundle"

This reverts commit 4abf8e6e

.

The curl process for updating the cookie file is not atomic.  When
fetching many bundles in parallel, we can sometimes corrupt the file
causing it to be cleared.  Since users should manage gitcookies on
their own, leave it read-only.

Bug: https://crbug.com/gerrit/12300
Change-Id: Id472c99b197bc4cf8533c649f8881509f38643c1
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254092
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>
(cherry picked from commit dc1d0e0c)