Fix multiple bugs in non-ownerdrawn wxListBox after recent merge.

Fix multiple bugs in non-ownerdrawn wxListBox after recent merge.

Changes done in ownerdraw-refactor branch have broken non-ownerdrawn list
boxes as the code was now using m_aItems array even for them but it's not used
in this case.

Also remove unnecessarily overridden Delete() method which didn't add anything
to the base class implementation but just deleted the same m_aItems pointer
twice.

Also use HasFlag(wxLB_OWNERDRAW) everywhere consistently instead of testing
for it manually.

Closes #11729.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63551 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775