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/SendSystemInfoDialog.cpp

@@ -481,7 +481,7 @@ static std::string generate_system_info_json()
         monitor_node.put("height", display.GetGeometry().GetHeight());
 
         // Only get the scaling on Win, it is not reliable on other platforms.
-        #if defined(_WIN32) && wxCHECK_VERSION(3, 1, 2)
+        #if defined(_WIN32)
             double scaling = display.GetPPI().GetWidth() / 96.;
             std::stringstream ss;
             ss << std::setprecision(3) << scaling;