fdo#75757: remove inheritance to std::vector

from StyleTreeArr_Impl.

Change-Id: I44621f913fc1b7ce0da9f41e4e0864edaea474b2
Reviewed-on: https://gerrit.libreoffice.org/11826
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>