Remove SDL3_main from build systems, remove most of src/main/*

XCode is still missing, and src/main/winrt/SDL3-WinRTResource*
still need to find a new home

Xcode/SDL/pkg-support/resources/CMake/sdl3-config.cmake

@@ -61,9 +61,4 @@ if(NOT TARGET SDL3::SDL3)
 endif()
 set(SDL3_SDL3_FOUND TRUE)
 
-if(NOT TARGET SDL3::SDL3_main)
-    add_library(SDL3::SDL3_main INTERFACE IMPORTED)
-endif()
-set(SDL3_SDL3_main_FOUND TRUE)
-
 check_required_components(SDL3)