indent: rip out old go 1.9 support

cmd/grpcurl/go1_10.go

@@ -1,10 +0,0 @@
-//go:build go1.10
-// +build go1.10
-
-package main
-
-func indent() string {
-	// In Go 1.10 and up, the flag package automatically
-	// adds the right indentation.
-	return ""
-}

cmd/grpcurl/go1_9.go

@@ -1,10 +0,0 @@
-//go:build !go1.10
-// +build !go1.10
-
-package main
-
-func indent() string {
-	// In Go 1.9 and older, we need to add indentation
-	// after newlines in the flag doc strings.
-	return "    \t"
-}

cmd/grpcurl/grpcurl.go

@@ -825,7 +825,7 @@ func prettify(docString string) string {
 		j++
 	}
 
-	return strings.Join(parts[:j], "\n"+indent())
+	return strings.Join(parts[:j], "\n")
 }
 
 func warn(msg string, args ...interface{}) {