Drop usr-is-merged from offline.downloads and installer-netinst

usr-is-merged is included in simple-cdd offline.downloads's since
version 0.6.9. Not sure why I needed to list it in installer-netinst as
well at the time, but after testing, I can confirm it's not needed
anymore.

kali-config/installer-netinst/packages

@@ -7,5 +7,3 @@
 # We still put kali-linux-firmware because we want firmware to be
 # in the ISO for use by the installer
 kali-linux-firmware
-# Add packages required by debootstrap (cf. #51)
-usr-is-merged

simple-cdd/profiles/offline.downloads

@@ -1,5 +1,3 @@
-# Add packages required by debootstrap (cf. #51)
-usr-is-merged
 # Add packages required for virtualization support (installed by
 # /usr/lib/finish-install.d/08hw-detect) including their recommends
 hyperv-daemons