[SPARK-19164][PYTHON][SQL] Remove unused UserDefinedFunction._broadcast

## What changes were proposed in this pull request?

Removes `UserDefinedFunction._broadcast` and `UserDefinedFunction.__del__` method.

## How was this patch tested?

Existing unit tests.

Author: zero323 <[email protected]>

Closes #16538 from zero323/SPARK-19164.

Author: zero323

python/pyspark/sql/functions.py

@@ -1826,7 +1826,6 @@ class UserDefinedFunction(object):
     def __init__(self, func, returnType, name=None):
         self.func = func
         self.returnType = returnType
-        self._broadcast = None
         self._judf = self._create_judf(name)
 
     def _create_judf(self, name):
@@ -1842,11 +1841,6 @@ def _create_judf(self, name):
             name, wrapped_func, jdt)
         return judf
 
-    def __del__(self):
-        if self._broadcast is not None:
-            self._broadcast.unpersist()
-            self._broadcast = None
-
     def __call__(self, *cols):
         sc = SparkContext._active_spark_context
         jc = self._judf.apply(_to_seq(sc, cols, _to_java_column))