PERF: Remove unnecessary copies in sorting functions (#33917)
- [x] closes #33917
- [x] tests passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This PR fixes pereformance regression after commit `dec736f3f` by removing unnecessary copies if `key=None` in sorting functions.

```
setup = """
import pandas as pd
import numpy as np
N = 10 ** 5
idx = pd.date_range(start="1/1/2000", periods=N, freq="s")
s = pd.Series(np.random.randn(N), index=idx)
"""

import timeit
timeit.timeit("s.sort_index()",setup=setup, number=100000)

# master
# 41.245621485984884
# now
# 14.826273362035863
```