sandbox: Drop use of special os_malloc() where possible

sandbox: Drop use of special os_malloc() where possible

Some sandbox files are not built with U-Boot headers, so with the renamed
malloc functions there is now no need to use the special os_... allocation
functions to access the system routines. Instead we can just call them
directly.

Update the affected files accordingly.

Signed-off-by: Simon Glass <sjg@chromium.org>