Remove spurious flutter pub add output.

[sigurdm]

I don't believe flutter pub add outputs this line when adding a dependency. In my local experiment it didn't.

Issues fixed by this PR (if any): Fixes #ISSUE-NUMBER

Presubmit checklist

• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

[sfshaza2]

Thanks, @sigurdm!