remove numpy_helper and some unneeded util functions
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22469?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@2be2ba5`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22469/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22469?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #22469   +/-   ##
=========================================
  Coverage          ?   92.04%           
=========================================
  Files             ?      169           
  Lines             ?    50740           
  Branches          ?        0           
=========================================
  Hits              ?    46705           
  Misses            ?     4035           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.45% <100%> (?)` | |
| #single | `42.24% <0%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22469?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/sparse/array.py](https://codecov.io/gh/pandas-dev/pandas/pull/22469/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL2FycmF5LnB5) | `91.38% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22469?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22469?src=pr&el=footer). Last update [2be2ba5...10e6205](https://codecov.io/gh/pandas-dev/pandas/pull/22469?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. any perf impact? > any perf impact?

It tentatively looks like the direct indexing is an improvement.

```
In [4]: arr = np.arange(100000)

In [5]: o = arr.astype('O')

In [6]: %timeit out = lib.astype_intsafe(o, 'i8')
```

Master:
100 loops, best of 3: 9.47 ms per loop
100 loops, best of 3: 16.3 ms per loop
100 loops, best of 3: 13.8 ms per loop

PR:
100 loops, best of 3: 8.65 ms per loop
100 loops, best of 3: 13.4 ms per loop
100 loops, best of 3: 7.6 ms per loop

```
In [14]: %timeit out = lib.astype_unicode(o)
```

Master:
10 loops, best of 3: 43.4 ms per loop
10 loops, best of 3: 63.1 ms per loop
10 loops, best of 3: 70.1 ms per loop
10 loops, best of 3: 68.2 ms per loop

PR:
10 loops, best of 3: 23.1 ms per loop
10 loops, best of 3: 41.4 ms per loop
10 loops, best of 3: 62.9 ms per loop
10 loops, best of 3: 60.4 ms per loop
 thanks!