Fix +build lines, remove extraneous pkg-config directive

This commit should make systray build on Ubuntu 20.04 and (hopefully) Debian
11 again. I fumbled the legacy "+build" tag syntax, apparently, and forgot
to remove a pkg-config directive that unconditionally required
libappindicator3 to be installed.

Author: Crosse

systray_linux_appindicator.go

@@ -1,3 +1,4 @@
+// +build linux,legacy_appindicator
 //go:build linux && legacy_appindicator
 
 package systray

systray_linux_ayatana.go

@@ -1,3 +1,4 @@
+// +build linux,!legacy_appindicator
 //go:build linux && !legacy_appindicator
 
 package systray

systray_nonwindows.go

@@ -1,14 +1,9 @@
 // +build !windows
+// go:build !windows
 
 package systray
 
-/*
-#cgo linux pkg-config: gtk+-3.0 appindicator3-0.1
-#cgo darwin CFLAGS: -DDARWIN -x objective-c -fobjc-arc
-#cgo darwin LDFLAGS: -framework Cocoa
-
-#include "systray.h"
-*/
+// #include "systray.h"
 import "C"
 
 import (