Clean up NVTabular examples

[bschifferer]

We want to develop new examples and clean the current NVTabular examples:

• Developing new examples for NVTabular : [Task] Developing new examples for NVTabular NVTabular#1541
• Remove/Move existing examples to other libraries [Task] Remove/Move existing examples to other libraries NVTabular#1542

[bschifferer]

CleanUp PR from Karl: NVIDIA-Merlin/NVTabular#1711