cmd/go/internal/get: remove unused tag selection code

selectTag has been hard coded to only understand the tag `go1` since
CL 6112060 which landed in 2012. The commit message asserted;

  Right now (before go1.0.1) there is only one possible tag,
  "go1", and I'd like to keep it that way.

Remove goTag and the unused matching code in selectTag.

Change-Id: I85f7c10f95704e22f8e8681266afd72bbcbe8fbd
Reviewed-on: https://go-review.googlesource.com/38112Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/get/get.go

@@ -10,9 +10,7 @@ import (
 	"go/build"
 	"os"
 	"path/filepath"
-	"regexp"
 	"runtime"
-	"strconv"
 	"strings"
 
 	"cmd/go/internal/base"
@@ -508,14 +506,6 @@ func downloadPackage(p *load.Package) error {
 	return nil
 }
 
-// goTag matches go release tags such as go1 and go1.2.3.
-// The numbers involved must be small (at most 4 digits),
-// have no unnecessary leading zeros, and the version cannot
-// end in .0 - it is go1, not go1.0 or go1.0.0.
-var goTag = regexp.MustCompile(
-	`^go((0|[1-9][0-9]{0,3})\.)*([1-9][0-9]{0,3})$`,
-)
-
 // selectTag returns the closest matching tag for a given version.
 // Closest means the latest one that is not after the current release.
 // Version "goX" (or "goX.Y" or "goX.Y.Z") matches tags of the same form.
@@ -531,56 +521,4 @@ func selectTag(goVersion string, tags []string) (match string) {
 		}
 	}
 	return ""
-
-	/*
-		if goTag.MatchString(goVersion) {
-			v := goVersion
-			for _, t := range tags {
-				if !goTag.MatchString(t) {
-					continue
-				}
-				if cmpGoVersion(match, t) < 0 && cmpGoVersion(t, v) <= 0 {
-					match = t
-				}
-			}
-		}
-
-		return match
-	*/
-}
-
-// cmpGoVersion returns -1, 0, +1 reporting whether
-// x < y, x == y, or x > y.
-func cmpGoVersion(x, y string) int {
-	// Malformed strings compare less than well-formed strings.
-	if !goTag.MatchString(x) {
-		return -1
-	}
-	if !goTag.MatchString(y) {
-		return +1
-	}
-
-	// Compare numbers in sequence.
-	xx := strings.Split(x[len("go"):], ".")
-	yy := strings.Split(y[len("go"):], ".")
-
-	for i := 0; i < len(xx) && i < len(yy); i++ {
-		// The Atoi are guaranteed to succeed
-		// because the versions match goTag.
-		xi, _ := strconv.Atoi(xx[i])
-		yi, _ := strconv.Atoi(yy[i])
-		if xi < yi {
-			return -1
-		} else if xi > yi {
-			return +1
-		}
-	}
-
-	if len(xx) < len(yy) {
-		return -1
-	}
-	if len(xx) > len(yy) {
-		return +1
-	}
-	return 0
 }