Revert "net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends"

This reverts commit 6bf32cda.

Various people have been reporting seeing "eth0: hw csum failure"
and callstacks dumped in the kernel log on 4.18, and since 4.14.71,
on both SMSC9514 and LAN7800 adapters.
This commit appears to be the reason, but potentially due to an
issue further down the stack. Revert whilst investigating the
trigger.

https://github.com/raspberrypi/linux/issues/2713
https://github.com/raspberrypi/linux/issues/2659
https://github.com/raspberrypi/linux/issues/2712



Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>