Move 'contrib' removal comment to git commit msg

Previous commit not only replaced 'non-free' with 'non-free-firmware',
it also removed the 'contrib' archive area from the sources.list.
It added a note about it in the code comments, which I think is the
wrong place. The 'why' of a change belongs in a git commit message,
where one can be as verbose as needed.
Code comments should be used to clarify the 'what' (it does) in case it
would not be immediately obvious.

The removal of 'contrib' totally makes sense though.
We did not use it and 'contrib' and 'non-free' are not part of
(official) Debian, whereas 'non-free-firmware' is now part of Debian
(official media) as a consequence of the change to the Debian Social
Contract following the GR vote.
With this change, we only use what Debian itself would only use.

Fixes: 1ffce8e6bb

generate-recipe.py

@@ -57,10 +57,6 @@ else:
 if suite == 'bookworm':
     firmware_component = 'non-free-firmware'
 else:
-    # We only need "non-free". "Contrib" was before included as well,
-    # as they are usually enabled together, but adding a second YAML
-    # element... makes the resulting code ugly and brittle. Besides,
-    # we don't use anything from contrib!
     firmware_component = 'non-free'
 
 # Extra wireless firmware: