Remove off-chain testing environment

[cmichi]

We discussed this in a number of ink! Alliance calls and came to the conclusion that we want to remove the off-chain testing environment.

We currently have three testing environments, in which contracts can be executed and tested:

• E2E test against a node process
• E2E test against a sandboxed pallet-revive
• Test against a simulation of pallet-revive that we came up with

The last one is what we'll remove.