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>

README.md

@@ -19,22 +19,20 @@ BSD-style license found in the LICENSE file.
 Official binary distributions are available at https://golang.org/dl/.
 
 After downloading a binary release, visit https://golang.org/doc/install
-or load [doc/install.html](./doc/install.html) in your web browser for installation
-instructions.
+for installation instructions.
 
 #### Install From Source
 
 If a binary distribution is not available for your combination of
 operating system and architecture, visit
-https://golang.org/doc/install/source or load [doc/install-source.html](./doc/install-source.html)
-in your web browser for source installation instructions.
+https://golang.org/doc/install/source
+for source installation instructions.
 
 ### Contributing
 
 Go is the work of thousands of contributors. We appreciate your help!
 
-To contribute, please read the contribution guidelines:
-	https://golang.org/doc/contribute.html
+To contribute, please read the contribution guidelines at https://golang.org/doc/contribute.html.
 
 Note that the Go project uses the issue tracker for bug reports and
 proposals only. See https://golang.org/wiki/Questions for a list of