Remove codepath asymmetry in dataframe count()
@jreback I noticed a codepath asymmetry in core.frame.count that leads to a substantial difference in dropna() performance depending on the axis. Using the path `df.dropna(axis=0)` takes yields a 2.5-5x improvement.

```
$ python vb_suite/test_perf.py -b upstream/master -t HEAD -r "dropna" -S -n 30
Invoked with :
--ncalls: 3
--repeats: 30


-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
frame_dropna_axis1_any                       |  18.5493 | 101.6117 |   0.1826 |
frame_dropna_axis1_all                       |  48.0193 | 128.8197 |   0.3728 |
frame_dropna_axis0_any                       |  17.0240 |  17.3127 |   0.9833 |
frame_dropna_axis0_all                       |  43.1127 |  43.3304 |   0.9950 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------

Ratio < 1.0 means the target commit is faster then the baseline.
Seed used: 1234

Target [84ad341] : Remove codepath asymmetry in dataframe count()
Base   [099a02c] : Merge pull request #9061 from behzadnouri/nan-pivot

pivot & unstack with nan in the index


                        count       mean        std       min        25%        50%        75%         max
frame_dropna_axis1_any      3  40.114509  54.044075  0.182551   9.365917  18.549283  60.080489  101.611694
frame_dropna_axis1_all      3  59.070599  64.932673  0.372764  24.196047  48.019330  88.419517  128.819704
frame_dropna_axis0_any      3  11.773351   9.345549  0.983328   9.003684  17.024040  17.168363   17.312686
frame_dropna_axis0_all      3  29.146001  24.379745  0.994976  22.053826  43.112675  43.221513   43.330352
```
