Remove wxCHECK_VERSION(3,1,x) guards and legacy wxinit.h macros

Since we now target wxWidgets 3.3, all wxCHECK_VERSION(3,1,N) checks
are always true. Remove the guards keeping only the true branches:

- I18N.hpp: Remove version guard around _wxGetTranslation_ctx macro
- ExtraRenderers.hpp, GUI_App.hpp: Simplify SUPPORTS_MARKUP to check
  only wxUSE_MARKUP (version check always true)
- ConfigWizard.cpp: Remove manual wxArrayInt comparison fallback
- SendSystemInfoDialog.cpp: Simplify display scaling guard to _WIN32 only
- GUI_Utils.cpp: Remove IsDark() fallback using luma approximation
- wxinit.h: Remove legacy wxEVT_BUTTON and wxEVT_HTML_LINK_CLICKED
  compat macros (these event names exist natively in wx 3.3)

src/slic3r/GUI/GUI_Utils.cpp

@@ -243,12 +243,7 @@ bool check_dark_mode() {
         return value <= 0;
     }
 #endif
-#if wxCHECK_VERSION(3,1,3)
     return wxSystemSettings::GetAppearance().IsDark();
-#else
-    const unsigned luma = wxGetApp().get_colour_approx_luma(wxSystemSettings::GetColour(wxSYS_COLOUR_WINDOW));
-    return luma < 128;
-#endif
 }