cookie: avoid mutex deadlock

... by removing the extra mutex locks around th call to
Curl_flush_cookies() which takes care of the locking itself already.

Bug: http://curl.haxx.se/mail/lib-2014-02/0184.html
test506: verify aa6884845168

After the fixed cookie lock deadlock, this test now passes and it
detects double-locking and double-unlocking of mutexes.