sandbox: sdl: Add stub sandbox_sdl_remove_display()

Building the sandbox with NO_SDL=1 resulted in an undefined reference to
'sandbox_sdl_remove_display'. Resolve this by adding a stub
implementation to match the stubs of the other similar functions.

Signed-off-by: Andrew Scull <ascull@google.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>