cmd/builder: update docs, delete more dead code

Updates golang/go#21191

Change-Id: I83319a4994959c31be2d41ec4622aac5f3ee4eca
Reviewed-on: https://go-review.googlesource.com/52632
Reviewed-by: Herbie Ong <herbie@google.com>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

cmd/builder/doc.go

@@ -3,36 +3,32 @@
 // license that can be found in the LICENSE file.
 
 /*
+The builder binary is the old continuous build client for the Go
+project. It is still used by a few builders, but most builders have
+been migrated to our newer build & test infrastructure.
 
-Go Builder is a continuous build client for the Go project.
-It integrates with the Go Dashboard AppEngine application.
+This command is intended to run continuously as a background process.
+It periodically pulls updates from the Go git repository and requests
+work to do from the Go Dashboard AppEngine application running at
+https://build.golang.org/.
 
-Go Builder is intended to run continuously as a background process.
-
-It periodically pulls updates from the Go Mercurial repository.
-
-When a newer revision is found, Go Builder creates a clone of the repository,
-runs all.bash, and reports build success or failure to the Go Dashboard.
-
-For a release revision (a change description that matches "release.YYYY-MM-DD"),
-Go Builder will create a tar.gz archive of the GOROOT and deliver it to the
-Go Google Code project's downloads section.
+When a new commit to test is found, the Go Builder creates a clone of
+the repository, runs all.bash, and reports build success or failure to
+the Go Dashboard.
 
 Usage:
 
-  gobuilder goos-goarch...
+  $ builder goos-goarch...
 
   Several goos-goarch combinations can be provided, and the builder will
   build them in serial.
 
 Optional flags:
 
-  -dashboard="godashboard.appspot.com": Go Dashboard Host
+  -dashboard="https://build.golang.org": Go Dashboard Host
     The location of the Go Dashboard application to which Go Builder will
     report its results.
 
-  -release: Build and deliver binary release archive
-
   -rev=N: Build revision N and exit
 
   -cmd="./all.bash": Build command (specify absolute or relative to go/src)
@@ -43,20 +39,12 @@ Optional flags:
 
   -v: Verbose logging
 
-  -external: External package builder mode (will not report Go build
-     state to dashboard or issue releases)
-
 The key file should be located at $HOME/.gobuildkey or, for a builder-specific
 key, $HOME/.gobuildkey-$BUILDER (eg, $HOME/.gobuildkey-linux-amd64).
 
 The build key file is a text file of the format:
 
   godashboard-key
-  googlecode-username
-  googlecode-password
-
-If the Google Code credentials are not provided the archival step
-will be skipped.
 
 */
 package main // import "golang.org/x/build/cmd/builder"