PERF: improve perf of float-based timeseries plotting
xref #15071

`_dt_to_float_ordinal` was already vectorized, so no need to map it on the index.

So now is 'irregular' plotting even faster as period-based plotting:

```
$ asv continuous upstream/master HEAD -b TimeseriesPlotting
...
· Running 4 total benchmarks (2 commits * 1 environments * 2 benchmarks)
[  0.00%] · For pandas commit hash 4fc0dca6:
[  0.00%] ·· Benchmarking conda-py2.7-Cython-matplotlib-numexpr-numpy-openpyxl-pytables-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt
[ 25.00%] ··· Running plotting.TimeseriesPlotting.time_plot_regular             126.57ms
[ 50.00%] ··· Running plotting.TimeseriesPlotting.time_plot_regular_compat      87.57ms
[ 50.00%] · For pandas commit hash de09c988:
[ 50.00%] ·· Benchmarking conda-py2.7-Cython-matplotlib-numexpr-numpy-openpyxl-pytables-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt
[ 75.00%] ··· Running plotting.TimeseriesPlotting.time_plot_regular             126.01ms
[100.00%] ··· Running plotting.TimeseriesPlotting.time_plot_regular_compat      305.58ms    

      before     after       ratio
  [de09c988] [4fc0dca6]
-  305.58ms    87.57ms      0.29  plotting.TimeseriesPlotting.time_plot_regular_compat

```
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15073?src=pr) is 84.75% (diff: 100%)
> Merging [#15073](https://codecov.io/gh/pandas-dev/pandas/pull/15073?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will not change coverage

```diff
@@             master     #15073   diff @@
==========================================
  Files           145        145          
  Lines         51151      51151          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43355      43355          
  Misses         7796       7796          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [db08da2...3971d56](https://codecov.io/gh/pandas-dev/pandas/compare/db08da2dfdb1740fdb67af3c29ab5867cd2602f8...3971d56ad0cc782a14d6d1d55d95f79e10469fb5?src=pr) lgtm. https://travis-ci.org/pandas-dev/pandas/jobs/190356982

I think this broke something on the slow 3.4 build ?