cmd/vet: remove pkgfact analyzer, left in by mistake

Also, document process for updating vendored x/tools.

Change-Id: I826744603ae0752e508a6db7334a2bf9adaf1289
Reviewed-on: https://go-review.googlesource.com/c/149963
Run-TryBot: Alan Donovan <adonovan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/vendor/README

@@ -19,3 +19,7 @@ make govendor work and will create the .cache folder in
 $GOROOT as a side-effect. Please make sure to delete
 the directory and not to include the directory in the
 commit by accident.
+
+The vendored copy of golang.org/x/tools is maintained by
+running the update-xtools.sh script in this directory,
+not by govendor.
\ No newline at end of file

src/cmd/vet/main.go

@@ -20,7 +20,6 @@ import (
 	"golang.org/x/tools/go/analysis/passes/loopclosure"
 	"golang.org/x/tools/go/analysis/passes/lostcancel"
 	"golang.org/x/tools/go/analysis/passes/nilfunc"
-	"golang.org/x/tools/go/analysis/passes/pkgfact"
 	"golang.org/x/tools/go/analysis/passes/printf"
 	"golang.org/x/tools/go/analysis/passes/shift"
 	"golang.org/x/tools/go/analysis/passes/stdmethods"
@@ -61,7 +60,6 @@ func main() {
 		loopclosure.Analyzer,
 		lostcancel.Analyzer,
 		nilfunc.Analyzer,
-		pkgfact.Analyzer,
 		printf.Analyzer,
 		shift.Analyzer,
 		stdmethods.Analyzer,