chore(windows): remove KMC_CHANGEUISTATE

The `KMC_CHANGEUISTATE` flag for `wm_keyman_control` is no longer
processed anywhere. Removes it and the `UpdateKeymanUI` function which
called it.

The `#define` for the flag has been left, commented out, to clarify what
would otherwise be a gap in identifiers (use `git blame` to learn more).