CLN/PERF: remove used functions; use C skip list for rolling median
removes some unused code
reverts this commit https://github.com/pydata/pandas/commit/a40226e5c4c3d29e5e2383433c5f0b94c3c0ecc1 

performance consideration

```
import pandas
import numpy
arr = numpy.random.rand(1000000)
%timeit pandas.rolling_median(arr, 1000)
```

master

```
1 loops, best of 3: 4.94 s per loop
```

branch

```
1 loops, best of 3: 821 ms per loop
```
