fdo#75757: remove inheritance to std::vector

Glyphs::iterator_pair_t was gone at df3c7e76cca60d246277731d0c525d5f7e6f6597.

Change-Id: I471a2ef659dd98527d2dca5fd8c3741851341ec5
Reviewed-on: https://gerrit.libreoffice.org/11533
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>