Replace old pow_with_uint with the new pow func

There was an old and barely used implementation of pow, which expected
both parameters to be uint and required more traits to be implemented.
Since a new implementation for `pow` landed, I'm proposing to remove
this old impl in favor of the new one.

The benchmark shows that the new implementation is faster than the one being removed:

```
    test num::bench::bench_pow_function               ..bench:      9429 ns/iter (+/- 2055)
    test num::bench::bench_pow_with_uint_function     ...bench:     28476 ns/iter (+/- 2202)
```