CLN: Remove tseries v0.19.0 deprecations
* Remove frequencies.get_standard_freq
* Drop the "freqstr" keyword from frequencies.to_offset

Deprecated in v0.19.0

xref #13874
rebase and push again, fixed some hanging by Travis CI @jreback : Surprised `master` is doing okay.  A lot of PR builds on Travis are getting hit with this "disk quota exceeded" error.  Seems to be affecting a lot of other repositories though. @gfyoung that was this morning. I think travis ci is having issues since then.  It's still happening though AFAICT.  The PR build before mine has already failed on two builds with that exact same error.

https://travis-ci.org/pandas-dev/pandas/builds/323891129 see the Travis Status: https://twitter.com/traviscistatus?ref_src=twsrc%5Egoogle%7Ctwcamp%5Eserp%7Ctwgr%5Eauthor

still happening. FYI this is the Red icon on the travis page # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19023?src=pr&el=h1) Report
> Merging [#19023](https://codecov.io/gh/pandas-dev/pandas/pull/19023?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c19bdc98fa3f856e2e6b3495bbab88b587801987?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19023/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19023?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19023      +/-   ##
==========================================
- Coverage   91.57%   91.57%   -0.01%     
==========================================
  Files         150      150              
  Lines       48941    48935       -6     
==========================================
- Hits        44817    44811       -6     
  Misses       4124     4124
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.93% <ø> (-0.01%)` | :arrow_down: |
| #single | `41.75% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19023?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/frequencies.py](https://codecov.io/gh/pandas-dev/pandas/pull/19023/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvZnJlcXVlbmNpZXMucHk=) | `93.9% <ø> (-0.15%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19023?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/19023?src=pr&el=footer). Last update [c19bdc9...f80c516](https://codecov.io/gh/pandas-dev/pandas/pull/19023?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback : Looks like Travis is up and running again.  All is green now.  PTAL. thanks!