Shibboleth:  Fix our implemtnation of CookieJar::deleteCookie

It was deleting too many cookies.
That function is virtual in Qt5 and is used when adding cookie.
But some Shibboleth have several cookies with the same name, and we need to keep them.

Our implementaiton was meant to delete all the shiboleth cookies when we want to log out

(cherry picked from commit 1f9d02e7fa6d4db57aeb01865503b046465f050a)

Conflicts:
	src/mirall/cookiejar.h