Do not set MULTILINE_COMMENT_PREFIXES before calling update-copyright

Do not set MULTILINE_COMMENT_PREFIXES before calling update-copyright

This was a local hack to work around a limitation in update-copyright.
But the limitation is now OBE, because the copyright statements should
now always fit on a single line, thanks to the new policy of using
one single year range in the copyright notice.

gdb/ChangeLog:

        * copyright.py (MULTILINE_COMMENT_PREFIXES): Delete.
        (update_files): Do not set MULTILINE_COMMENT_PREFIXES
        environment variable before calling update-copyright.