Core: remove sdlew/WITH_SDL_DYNLOAD & disable SDL by default

Disable dynamic SDL loading as well as disable SDL for release builds.

This was only used for audio output which can already use OpenAL
if there are back-ends not natively supported by Blender.

- Remove extern/sdlew/
- Remove the WITH_SDL_DYNLOAD build option.
- Remove `bpy.app.sdl.available`.

Ref !127554

intern/audaspace/CMakeLists.txt

@@ -28,13 +28,6 @@ if(NOT WITH_SYSTEM_AUDASPACE)
   list(APPEND LIB
     audaspace
   )
-  if(WITH_SDL)
-    if(WITH_SDL_DYNLOAD)
-      list(APPEND LIB
-        extern_sdlew
-      )
-    endif()
-  endif()
 else()
   list(APPEND LIB
     ${AUDASPACE_C_LIBRARIES}