Remove unnecessary CommandStateChangeConstants enum redeclaration.

This declaration conflicted with the one in the latest MinGW-W64 headers and
wasn't needed anyhow as we don't use neither the enum itself nor its members
in our code, so just remove it to fix the build in 64 bits with MinGW-W64 4.8.

This is the backport of r75669 from trunk.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75775 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775