Also free internal handlers when wxMSW wxStaticBitmap is destroyed.

r76141 fixed the resource leak when wxStaticBitmap image was replaced by
another one but the leak still happened at the end, when the wxStaticBitmap
was destroyed.

Fix it there as well in the same way.

Closes #16068.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76145 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775