[SPARK-3340] Deprecate ADD_JARS and ADD_FILES

I created a patch that disables the environment variables.
Thereby scala or python shell log a warning message to notify user about the deprecation
with the following message:
scala: "ADD_JARS environment variable is deprecated, use --jar spark submit argument instead"
python: "Warning: ADD_FILES environment variable is deprecated, use --py-files argument instead"

Is it what is expected or the code associated with the variables should be just completely removed?
Should it be somewhere documented?

Author: azagrebin <azagrebin@gmail.com>

Closes #4616 from azagrebin/master and squashes the following commits:

bab1aa9 [azagrebin] [SPARK-3340] Deprecate ADD_JARS and ADD_FILES: minor readability issue
0643895 [azagrebin] [SPARK-3340] Deprecate ADD_JARS and ADD_FILES: add warning messages
42f0107 [azagrebin] [SPARK-3340] Deprecate ADD_JARS and ADD_FILES