[VTA] Removing dependences to NNVM

[tmoreau89]

This PR removes all dependences to NNVM, and deletes deprecated libraries that relied upon analyzing NNVM graphs.

[liangfu]

@tmoreau89 Thanks for deprecating nnvm in VTA.

Would you please also fix the following line error
https://github.com/apache/incubator-tvm/blob/dabde40f15423fc9632824dcd95696f36cf17a01/vta/tutorials/frontend/deploy_vision_on_vta.py#L173
, where relay.quantize.quantize is now defined to accept mod directly, see
https://github.com/apache/incubator-tvm/blob/dabde40f15423fc9632824dcd95696f36cf17a01/python/tvm/relay/quantize/quantize.py#L313-L322
for your reference.

[tmoreau89]

@liangfu thank you for reporting the bug; I believe that #4295 has broken front end support in Relay for VTA. The issue is that this wasn't caught by the CI which should have re-built the VTA tutorials sphynx gallery. I'm following up to make sure that this doesn't happen in the future. I'll address your request in a separate PR.