fdo#75757: remove inheritance to std::vector

from SwGrfFmtColls.

Change-Id: I4dd83917584406704d1a52f85b8973f2714e9d0b
Reviewed-on: https://gerrit.libreoffice.org/11731
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>