Fix toolbar repainting after deleting a tool in wxMSW.

The toolbar was in an inconsistent state when recalculating the sizes of the
separators used as placeholders for the controls and stretch spacers as the
tool was already deleted from the native toolbar but still present in wx
internal toolbar data and this resulted in discrepancies between the indices
in the native and wx toolbars.

Fix this by specially marking the already deleted but not yet removed tool and
ignoring it during the recalculations.

This makes the old, and only partially successful, attempt to work around this
bug in DoDeleteTool() unnecessary and so the code is actually simplified by
this change.

Closes #16095.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76183 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775