[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.

bin/pyspark

@@ -30,14 +30,6 @@ export _SPARK_CMD_USAGE="Usage: ./bin/pyspark [options]"
 # (e.g. PYSPARK_DRIVER_PYTHON_OPTS='notebook').  This supports full customization of the IPython
 # and executor Python executables.
 
-# Determine the Python executable to use if PYSPARK_PYTHON or PYSPARK_DRIVER_PYTHON isn't set:
-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
-
 # Fail noisily if removed options are set
 if [[ -n "$IPYTHON" || -n "$IPYTHON_OPTS" ]]; then
   echo "Error in pyspark startup:" 
@@ -47,10 +39,10 @@ fi
 
 # Default to standard python interpreter unless told otherwise
 if [[ -z "$PYSPARK_DRIVER_PYTHON" ]]; then
-  PYSPARK_DRIVER_PYTHON="${PYSPARK_PYTHON:-"$DEFAULT_PYTHON"}"
+  PYSPARK_DRIVER_PYTHON="${PYSPARK_PYTHON:-"python"}"
 fi
 
-WORKS_WITH_IPYTHON=$($DEFAULT_PYTHON -c 'import sys; print(sys.version_info >= (2, 7, 0))')
+WORKS_WITH_IPYTHON=$(python -c 'import sys; print(sys.version_info >= (2, 7, 0))')
 
 # Determine the Python executable to use for the executors:
 if [[ -z "$PYSPARK_PYTHON" ]]; then
@@ -58,7 +50,7 @@ if [[ -z "$PYSPARK_PYTHON" ]]; then
     echo "IPython requires Python 2.7+; please install python2.7 or set PYSPARK_PYTHON" 1>&2
     exit 1
   else
-    PYSPARK_PYTHON="$DEFAULT_PYTHON"
+    PYSPARK_PYTHON=python
   fi
 fi
 export PYSPARK_PYTHON