doc: remove install.html and install-source.html

These live in x/website/content/doc now.
The copies here just attract edits that have no actual effect.

For #40496.
For #41861.

Change-Id: I2fdd7375e373949eb9a88f4cdca440b6a5d45eea
Reviewed-on: https://go-review.googlesource.com/c/go/+/291709


Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>