Fix removal of cookies without a Domain field

Summary:
Cookies without a Domain field weren't removed from KCookieServer because
WebEnginePartCookieJar::removeCookie called KCookieServer::deleteCookie passing
the whole URL as second argument instead of only passing the host. This had no
effect on cookies with a Domain field, but prevented KCookieServer to delete it
in case the cookie had no domain field.

This bug had visible consequences, for example when logging in GMail. With the
previous behavior, the following steps lead to a "Cookie Mismatch" message from
GMail:
- log into GMail
- log out from GMail
- quit Konqueror
- restart Konqueror
- attempt to log into GMail

With this fix, the second attempt to log into GMail succeeds.

Test Plan:
Run autotests. Try the steps described above if you have a GMail
account

Reviewers: dfaure

Reviewed By: dfaure

Differential Revision: https://phabricator.kde.org/D22366