Revert "ipv4: ip_tunnel: Unmask upper DSCP bits in ip_tunnel_xmit()"

This reverts commit 9b8f85c4 which is
commit c2b639f9 upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Icf10739c4f21eebdf3d782ac44dc1bab179d6a8b
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>