Since we now target wxWidgets 3.3, the custom DPI change event
workaround (DpiChangedEvent, EVT_DPI_CHANGED_SLICER,
register_win32_dpi_event) is dead code. wxWidgets 3.1.3+ provides
native wxEVT_DPI_CHANGED / wxDPIChangedEvent which is already
wired up in the "true" branch of the version guards.
Removes:
- DpiChangedEvent struct and EVT_DPI_CHANGED_SLICER declaration/definition
- register_win32_dpi_event() function and its call site
- All associated #if !wxVERSION_EQUAL_OR_GREATER_THAN(3,1,3) guards