[BUGFIX] Cleanly unset cookies on login in cookie-check

Needed to workaround a login-problem with IE11.

ExtJS tries to clear a cookie with different settings than when
setting the cookie. In IE11 this leads to problems with the cookie
being set twice on the next call to set(). The get() however
would return the first (empty) cookie.

Using set() with a date in the past also clears the cookie but
will correctly use the same path-settings.

Change-Id: Ieff22129895cd89ca2e1429703daf1636596ecb6
Resolves: #53818
Releases: 6.2, 6.1, 6.0, 4.7, 4.5
Reviewed-on: https://review.typo3.org/25870
Reviewed-by: Steffen Ritter
Reviewed-by: Markus Klein
Tested-by: Markus Klein