Fixed bug 3789 - Android : small clean up

Sylvain

Since https://hg.libsdl.org/SDL/rev/6546daa45a02
SDL_android_main.c is empty and then produce a warning
nativeInit does not exist and dont need to be mark undefined

CMakeLists.txt

@@ -876,7 +876,6 @@ if(ANDROID)
 
   CheckPTHREAD()
 
-  list(APPEND EXTRA_LDFLAGS "-Wl,--undefined=Java_org_libsdl_app_SDLActivity_nativeInit")
 endif()
 
 # Platform-specific options and settings