[SPARK-16399][PYSPARK] Force PYSPARK_PYTHON to python

## What changes were proposed in this pull request?

I would like to change

```bash
if hash python2.7 2>/dev/null; then
  # Attempt to use Python 2.7, if installed:
  DEFAULT_PYTHON="python2.7"
else
  DEFAULT_PYTHON="python"
fi
```

to just ```DEFAULT_PYTHON="python"```

I'm not sure if it is a great assumption that python2.7 is used by default, when python points to something else.

## How was this patch tested?

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)

(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

Author: MechCoder <mks542@nyu.edu>

Closes #14016 from MechCoder/followup.