fdo#75757: remove inheritance to std::vector

from SwLayCacheImpl.

Change-Id: I77520495aab1d6a572b25433396202b11e478636
Reviewed-on: https://gerrit.libreoffice.org/11158
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>