go/analysis/passes/buildtag: retire Go 1.15 support

Go 1.17 is long since out.

Furthermore, CL 516575 added a use of parser.SkipObjectResolution to
a file with a go1.16 build constraint. This used to be fine, but not
now that vet checks for this (see go.dev/issue/66092) since that API
is new to Go 1.17.

For golang/go#66092.

Change-Id: I749b2aa52e02308415c27028be15a436ee11d95c
Reviewed-on: https://go-review.googlesource.com/c/tools/+/593297
Reviewed-by: Michael Matloob <[email protected]>
Reviewed-by: Sam Thanawalla <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: dmitshur

go/analysis/passes/buildtag/buildtag.go

@@ -2,9 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.16
-// +build go1.16
-
 // Package buildtag defines an Analyzer that checks build tags.
 package buildtag
 

go/analysis/passes/buildtag/buildtag_old.go

@@ -5,8 +5,6 @@
 package buildtag_test
 
 import (
-	"runtime"
-	"strings"
 	"testing"
 
 	"golang.org/x/tools/go/analysis"
@@ -15,9 +13,6 @@ import (
 )
 
 func Test(t *testing.T) {
-	if strings.HasPrefix(runtime.Version(), "go1.") && runtime.Version() < "go1.16" {
-		t.Skipf("skipping on %v", runtime.Version())
-	}
 	analyzer := *buildtag.Analyzer
 	analyzer.Run = func(pass *analysis.Pass) (interface{}, error) {
 		defer func() {