Test suite: remove XXX from two other scenarios (refs: #16004).

These scenarios are taking the expected shortcut (installing from the Tails
USB image directy), due to the following chain of parent checkpoints:
 - usb-install-with-persistence-logged-in
 - usb-install-with-persistence-tails-greeter
 - usb-install-logged-in
 - usb-install-tails-greeter

features/usb_install.feature

@@ -56,14 +56,13 @@ Feature: Installing Tails to a USB drive
     But there is no persistence partition on USB drive "install"
 
   Scenario: Re-installing Tails over an existing USB installation with a persistent partition
-    # XXX: We reach this first checkpoint only to ensure that the ' __internal' disk has reached the state (Tails installed + persistent partition set up) we need before we clone it below.
-    # XXX: do this "Given" without Tails Installer
+    # We reach this first checkpoint only to ensure that the ' __internal' disk has reached the state (Tails installed + persistent partition set up) we need before we clone it below.
+    # This first part is done without Tails Installer (install from Tails USB image)
     Given I have started Tails without network from a USB drive with a persistent partition enabled and logged in
     Given I have started Tails from DVD without network and logged in
     And I clone USB drive "__internal" to a temporary USB drive "install"
     And I plug USB drive "install"
-    # XXX: keep doing reinstall with Tails Installer: that's what this scenario
-    # is about
+    # This second part is done with Tails Installer, that's what this scenario is about
     When I reinstall Tails to USB drive "install" by cloning
     Then the running Tails is installed on USB drive "install"
     And there is no persistence partition on USB drive "install"

features/usb_upgrade.feature

@@ -134,7 +134,6 @@ Feature: Upgrading an old Tails USB installation
     And all persistent directories from the old Tails version have safe access rights
 
   Scenario: Upgrading Tails with Tails Upgrader through an incremental upgrade
-    # XXX: ensure we do this Given without Tails Installer (usb-install-tails-greeter)
     Given I have started Tails without network from a USB drive with a persistent partition enabled and logged in
     And Tails is fooled to think it is running version 1.0~test
     And the file system changes introduced in version 1.1~test are not present