remove comments on copyright status from UTF-8 implementation files

remove comments on copyright status from UTF-8 implementation files

despite clarifications made to the COPYRIGHT file in commit
f0a61399330bae42beeb27d6ecd05570b3382a60, there continues to be
confusion about whether the permissions granted actually apply to all
files. I am the sole author of these files and clearly intend, and
have always intended, for the grant of permission to apply to them.