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/wxinit.h

@@ -12,14 +12,4 @@
 // later when we no longer need to undef _
 #define __(s)                     wxGetTranslation((s))
 
-// legacy macros
-// https://wiki.wxwidgets.org/EventTypes_and_Event-Table_Macros
-#ifndef wxEVT_BUTTON
-#define wxEVT_BUTTON wxEVT_COMMAND_BUTTON_CLICKED
-#endif
-
-#ifndef wxEVT_HTML_LINK_CLICKED
-#define wxEVT_HTML_LINK_CLICKED wxEVT_COMMAND_HTML_LINK_CLICKED
-#endif
-
 #endif