remove redundant build-tag comments

- The "linux" build tags were redundant as they were in a _linux file
- The project already states that go1.10 is a minimum (and long obsolete,
  so unlikely to be used still).
- Format the remaining build-tags for current go versions.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

README.md

@@ -23,6 +23,7 @@ import (
     "fmt"
     "net"
     "runtime"
+
     "github.com/vishvananda/netns"
 )
 

netns.go

@@ -71,7 +71,7 @@ func (ns *NsHandle) Close() error {
 	if err := unix.Close(int(*ns)); err != nil {
 		return err
 	}
-	(*ns) = -1
+	*ns = -1
 	return nil
 }
 

netns_linux.go

@@ -1,6 +1,3 @@
-//go:build linux && go1.10
-// +build linux,go1.10
-
 package netns
 
 import (

netns_unspecified.go

@@ -1,3 +1,4 @@
+//go:build !linux
 // +build !linux
 
 package netns