given(): remove support for lexical $_

given(): remove support for lexical $_

There is dead code that used to allow

    my $_;
    ...
    given ($foo) {
        # lexical $_ aliased to $foo here
    }

Now that lexical $_ has been removed, remove the code.  I've left the
signatures of the newFOO() functions unchanged; they just expect a target
of 0 to always be passed now.