[BUGFIX] Multiple Set-Cookie headers for deleting FE cookie

In the current master every time when removeCookie is triggered a
Set-Cookie header is generated. A cookie should only be deleted if it
was set before. As removeCookie can handle different cookie names, the
check is performed before calling removeCookie function.

Resolves: #56733
Releases: 6.2
Change-Id: I518220ce2def8d9db901b1e8daa87de745317629
Reviewed-on: https://review.typo3.org/28227
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Oliver Klee
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert