Get rid of wxAuiToolBar::m_style and just use base class m_windowStyle.

Get rid of wxAuiToolBar::m_style and just use base class m_windowStyle.

This fixes an assert exposed by the addition of wxAuiToolBar::Create() in
r72785: as m_style was not initialized before, calling GetWindowStyle() from
wxControl::Create() returned wrong flags.

Fix this by just removing m_style completely, there doesn't seem to be any
need for it nor for overriding GetWindowStyleFlag().

See #13520.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72816 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775