doc: remove root.html and conduct.html (moved to x/website)

root.html has been moved to the x/website repo in CL 180959
(commit golang/website@d83058c).

conduct.html has been moved to the x/website repo in CL 207437
(commit golang/website@99763cb).

There should be only one copy, otherwise it may lead to confusion,
or changes made in the wrong place. This CL removes the old copies
from this repo since they're no longer used.

Updates #29206

Change-Id: I41adfb2c34ed3d870fb7a671f48ccc8f90863feb
Reviewed-on: https://go-review.googlesource.com/c/go/+/213157
Run-TryBot: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Jay Conrod <[email protected]>