disable bundled SDL

Index: lib/CMakeLists.txt
--- lib/CMakeLists.txt.orig
+++ lib/CMakeLists.txt
@@ -15,8 +15,6 @@ if (FSO_BUILD_WITH_OPENGL)
 	add_subdirectory(opengl)
 endif()
 
-ADD_SUBDIRECTORY(libsdl)
-
 ADD_SUBDIRECTORY(jansson)
 # These are only required for the web-ui which is non-Windows
 IF(UNIX)
