Update image-classification benchmark setup

[rahulchaphalkar]

Addresses #267
Combines 2-step setup into 1-step, now only source ./benchmarks/image-classification/setup.sh is needed.
Updated Readme to reflect this.
@jlb6740

[jlb6740]

@rahulchaphalkar When I tried to run this, I got this error:

"[setupvars.sh] ERROR: Unsupported Python version. Please install one of Python 3.6 - 3.9 (64-bit) from https://www.python.org/downloads/"

This is coming from the OpenVino download where I usually just go in and comment out the lines myself. But it got me thinking if this is the latest version of OpenVino that will work here?

[rahulchaphalkar]

But it got me thinking if this is the latest version of OpenVino that will work here?

The openvino version here is old, 2022.x, as is the wasi-nn bindings version, 0.2.1. I haven't really updated anything in this PR, just tried to smooth over some setup script issues with openvino.

"[setupvars.sh] ERROR: Unsupported Python version. Please install one of Python 3.6 - 3.9

As you pointed out this seems to be coming from setupvars.sh script from openvino. I think this should pop up irrespective of the changes in this PR right? Seems like it just needs an updated python to set up libopenvino.so path and stuff.