Revert "net: tun: Implement TUN_NOCHECKSUM"

This reverts commit commit 710d10e1 ("net: tun: Implement
TUN_NOCHECKSUM").

This change is no longer needed as it may blindly skip checksum
validation for packets whose checksum may not have been validated
earlier in the stack.

Hardware supports checksum offload for IPv4 / IPv6 TCP / UDP packets
only. The original commit would have allowed ICMP / ICMPv6 /
fragmented packets to skip checksum validation completely in the
network stack which is incorrect.

Change-Id: I88b52329edc89e252f1d55c9741eda4c8e3cffd8
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>