No, it is not legal.
The only thing which you are allowed to change/remove is the smf version number, which can be edited in index.php
the entire line
Powered by SMF 2.0 RC3 | SMF © 2006–2010, Simple Machines LLC
must remain unaltered (with the exception of 2.0RC3, as already stated)
Why do you want to remove that anyway?
Plus... you put your question in the SITE COMMENTS board (which is for comments about the simplemachines.org site)