Removed extern "C" linkage from main() declaration

According to the C++ spec, main() should not have any linkage specifier:
https://en.cppreference.com/w/cpp/language/main_function

Fixes https://github.com/libsdl-org/SDL/issues/8738

include/SDL3/SDL_main_impl.h

@@ -72,12 +72,6 @@ int SDL_main(int argc, char **argv)
 #define WINAPI __stdcall
 #endif
 
-#include <SDL3/SDL_begin_code.h>
-
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 typedef struct HINSTANCE__ * HINSTANCE;
 typedef char* LPSTR;
 typedef wchar_t* PWSTR;
@@ -119,12 +113,6 @@ int WINAPI WinMain(HINSTANCE hInst, HINSTANCE hPrev, LPSTR szCmdLine, int sw)
     return SDL_RunApp(0, NULL, SDL_main, NULL);
 }
 
-#ifdef __cplusplus
-} /* extern "C" */
-#endif
-
-#include <SDL3/SDL_close_code.h>
-
 /* end of __WIN32__ and __GDK__ impls */
 #elif defined(__WINRT__)
 
@@ -196,23 +184,11 @@ TInt E32Main()
 
 #else /* platforms that use a standard main() and just call SDL_RunApp(), like iOS and 3DS */
 
-#include <SDL3/SDL_begin_code.h>
-
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 int main(int argc, char *argv[])
 {
     return SDL_RunApp(argc, argv, SDL_main, NULL);
 }
 
-#ifdef __cplusplus
-} /* extern "C" */
-#endif
-
-#include <SDL3/SDL_close_code.h>
-
 /* end of impls for standard-conforming platforms */
 
 #endif /* __WIN32__ etc */