remove positive kernel check from powdenest

It is not always true that (p**x)**y = p**(x*y) if p is positive.
It depends on the value of x. e.g.
>>> e = 3 + I
>>> r = Rational(1, 3)
>>> b = .02
>>> ((b**e).n()**r).n(2)
0.014 + 0.014*I
>>> (b**(e*r)).n(2)
0.0053 - 0.019*I