Remove wxTB_FLAT from wxTB_DEFAULT_STYLE.

The actual default styles for the toolbars don't include wxTB_FLAT anywhere,
so don't include it in the (currently almost unused) wxTB_DEFAULT_STYLE
neither.

See #16667.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78097 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Use wxTB_DEFAULT_STYLE as default value for wxToolBar style.

No real changes, but this is more consistent with the other controls.

It also highlights the ports which use non-default style, e.g. wxQt where
wxNO_BORDER should probably not be used as part of default toolbar style but
rather added in the ctor implementation.

Closes #16667.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78098 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775