x86/syscall/compat: Remove ia32_unistd.h

This header is now just a wrapper for unistd_32_ia32.h.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240321211847.132473-3-brgerst@gmail.com