Remove unnecessary default from a switch statement.

Remove unnecessary default from a switch statement.

Don't check for impossible enum element value, it's better to not have the
default clause at all to get a warning from g++ if this enum ever changes
later. And for now the presence of default results in a warning from Coverity
about unreachable code.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73147 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775