fdo#75757: Remove inheritance from std::vector

Problem: Many classes inherit directly from std::vector
See comment#6 by kendy for more details.

Solution: Removed class SwpHstry altogether and moved its
destructor's functionality to SwHistory's destructor.
Changed type of m_SwpHstry from SwpHstry to vector<SwHistoryHint*>

Change-Id: I32a7e111fef7c5e7b83ecee6469e173b5a21a5bf
Reviewed-on: https://gerrit.libreoffice.org/9725
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>