Don't use C++ streams in wxString::FromCDouble() implementation.

This doesn't work if the global C++ locale had been changed and we can't call
imbue(locale::classic()) to ensure that the stream we use here uses the C
locale because imbue() is hopelessly broken in some implementations.

So just get rid of this code and keep only the hack replacing the decimal
separator with the dot explicitly. This is ugly but should always work in
practice and is also consistent with ToCDouble().

Closes #16343.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76791 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775