Perl_sv_vcatpvfn_flags: remove redundant code

Perl_sv_vcatpvfn_flags: remove redundant code

At the start of the function, it marks the output as being utf8 if the
first arg is utf8. But this should be taken care of when the individual
args (including the first one are processed). So its redundant code.

In fact it would sometimes cause the resultant string to be unnecessarily
upgraded to utf8, e.g.:

    my $precis = "9";
    utf8::upgrade($precis);
    my $s = sprintf "%.*f\n", $precis, 1.1;
    # whoops, $s is now utf8