Deprecate or Re-implement support for user tarballs (used by CRAB)

[khurtado]

Impact of the new feature
Simplify cmssw step deprecating support of python-based user tarballs unpacking.

Is your feature request related to a problem? Please describe.
We use some python-based code living in WMCore in the bash script we use to execute cmsRun. We want to decouple cmsRun from WMCore and this script that lives in WMCore, would be executed using the CMSSW python version

Describe the solution you'd like
We don't use this in production, but we should make sure CRAB is not using it either so we can deprecate its use. Otherwise, we could likely re-implement this script in full-bash, so it doesn't depend on any python version.

Note: This is where CRAB seems to use it.
https://github.com/dmwm/CRABServer/blob/dc26ef6512e5c75cf7ce0106f9838e63b870dac1/scripts/CMSRunAnalysis.py#L886
https://github.com/dmwm/CRABServer/blob/dc26ef6512e5c75cf7ce0106f9838e63b870dac1/scripts/CMSRunAnalysis.py#L740

Additional context
Although this is sort of related to python2/3 migration, it does not affect production but CRAB, as we don't call this part of the code in production, so we can deal with this later on

[khurtado]

As mentioned in:

dmwm/CRABServer#7215 (comment)

This is not used by CRAB anymore and we can now get rid of these lines:

WMCore/src/python/WMCore/WMSpec/Steps/Executors/CMSSW.py

Lines 441 to 454 in a746fe3

	if [ -n "$USER_TARBALL" ] ; then
	python2 -m WMCore.WMRuntime.Scripts.UnpackUserTarball $USER_TARBALL $USER_FILES
	EXIT_STATUS=$?
	if [ $EXIT_STATUS -ne 0 ]; then
	echo "***\nCouldn't untar sandbox with python2: $EXIT_STATUS\n";
	echo "***\nWill try with python2.6 as it might be an old CMSSW release!"
	python2.6 -m WMCore.WMRuntime.Scripts.UnpackUserTarball $USER_TARBALL $USER_FILES
	EXIT_STATUS=$?
	if [ $EXIT_STATUS -ne 0 ]; then
	echo "***\nCouldn't untar sandbox with python2.6: $EXIT_STATUS\n";
	exit 74;
	fi
	fi
	fi