doc: remove last pieces of advice to set GOROOT

install.html still insisted that GOROOT must be set if a binary install
of Go is set up in a custom directory. However, since 1.10, this has
been unnecessary as the GOROOT will be found based on the location of
the 'go' binary being run.

Likewise, install-source.html includes an 'export GOROOT' line in a
section that only talks about explicitly setting GOARCH and GOOS, which
is optional. We don't want to have users think it is recommended to set
GOROOT here either, so remove the unnecessary line.

Change-Id: I7dfef09f9a1d003e0253b793d63ea40d5cf1837f
Reviewed-on: https://go-review.googlesource.com/c/161758
Reviewed-by: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

doc/install-source.html

@@ -666,7 +666,6 @@ something like this:
 </p>
 
 <pre>
-export GOROOT=$HOME/go1.X
 export GOARCH=amd64
 export GOOS=linux
 </pre>