gobuild: drop support for building on Go < 1.10 (#2960)

Remove code to build on old versions of Go that didn't support the all=
query for -gcflags.

pkg/gobuild/gobuild.go

@@ -11,7 +11,6 @@ import (
 	"time"
 
 	"github.com/go-delve/delve/pkg/config"
-	"github.com/go-delve/delve/pkg/goversion"
 )
 
 // Remove the file at path and issue a warning to stderr if this fails.
@@ -33,25 +32,6 @@ func Remove(path string) {
 	}
 }
 
-// optflags generates default build flags to turn off optimization and inlining.
-func optflags(args []string) []string {
-	// after go1.9 building with -gcflags='-N -l' and -a simultaneously works.
-	// after go1.10 specifying -a is unnecessary because of the new caching strategy,
-	// but we should pass -gcflags=all=-N -l to have it applied to all packages
-	// see https://github.com/golang/go/commit/5993251c015dfa1e905bdf44bdb41572387edf90
-
-	ver, _ := goversion.Installed()
-	switch {
-	case ver.Major < 0 || ver.AfterOrEqual(goversion.GoVersion{Major: 1, Minor: 10, Rev: -1}):
-		args = append(args, "-gcflags", "all=-N -l")
-	case ver.AfterOrEqual(goversion.GoVersion{Major: 1, Minor: 9, Rev: -1}):
-		args = append(args, "-gcflags", "-N -l", "-a")
-	default:
-		args = append(args, "-gcflags", "-N -l")
-	}
-	return args
-}
-
 // GoBuild builds non-test files in 'pkgs' with the specified 'buildflags'
 // and writes the output at 'debugname'.
 func GoBuild(debugname string, pkgs []string, buildflags string) error {
@@ -85,7 +65,7 @@ func goBuildArgs(debugname string, pkgs []string, buildflags string, isTest bool
 	if isTest {
 		args = append([]string{"-c"}, args...)
 	}
-	args = optflags(args)
+	args = append(args, "-gcflags", "all=-N -l")
 	if buildflags != "" {
 		args = append(args, config.SplitQuotedFields(buildflags, '\'')...)
 	}