Fix mouse wheel event coordinates in wxFrame in wxMSW.

The screen to client conversion for this event coordinates took the toolbar
height into account twice, resulting in a wrong value if the event was handled
in a frame that did have a toolbar.

Closes #15812.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75559 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775