DEPR: Remove legacy offsets
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Follow-up to #13590. Remove legacy offset aliases that remained in `pandas/tseries/frequencies.py`: `_period_alias_dictionary()` and `_period_alias_dict`.

@sinhrks 

hmm, were these previously deprecated?
 @jreback All the removed aliases raise a `FutureWarning` in 0.18.1 and a `ValueError` in `master` &mdash; at least when creating `Period` and `PeriodIndex` objects, as well as calling `.asfreq()` on them.
 @agraboso ok, guess weren't tested!

@sinhrks ?
 The are a few tests &mdash; see, for example,
- [test_period.py#L444-L463](https://github.com/pydata/pandas/blob/master/pandas/tseries/tests/test_period.py#L444-L463)
- [test_period.py#L689-L691](https://github.com/pydata/pandas/blob/master/pandas/tseries/tests/test_period.py#L689-L691)
- [test_period.py#L1148-L1165](https://github.com/pydata/pandas/blob/master/pandas/tseries/tests/test_period.py#L1148-L1165)
- [test_offsets.py#L4561-L4565](https://github.com/pydata/pandas/blob/master/pandas/tseries/tests/test_offsets.py#L4561-L4565)

But I don't know if there may be some edge cases in which the removed aliases could be accepted.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13868?src=pr) is 85.22% (diff: 100%)

> Merging [#13868](https://codecov.io/gh/pydata/pandas/pull/13868?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **0.01%**

``` diff
@@             master     #13868   diff @@
==========================================
  Files           140        140          
  Lines         50456      50398    -58   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43007      42950    -57   
+ Misses         7449       7448     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [d4f95fd...25d932d](https://codecov.io/gh/pydata/pandas/compare/d4f95fdebb74db5edb09c743b48c4b03e0940591...25d932d09549e9050211c69fcd72d14914d92093?src=pr)
 Yes it has been missed as not tested. Thanks for the catch!

NOTE: I found a condition which doesn't show warning. I think it is rare and can't be a reason to keep the impl, but may need discussion.

```
pd.Period(ordinal=1, freq='SECONDLY')
# Period('1970-01-01 00:00:01', 'S')
```
 @sinhrks `pd.Period(ordinal=1, freq='SECONDLY')` raises a `FutureWarning` in 0.18.1. I added a test of `pd.Period(ordinal=1, freq='SECONDLY')` (and other removed aliases) that fails on `master` but passes with this PR.
 @agraboso You're right, confirmed using 0.18.1 to show warning.
 thanks!
