[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 <zero323@users.noreply.github.com>

Closes #16538 from zero323/SPARK-19164.