remove forkOverride from test runner and processTransaction

[jangko]

The early general state test is using fork override because at that time there is no proper hard fork management.
It was handy at that time, but now it becomes unnecessarily complicated.

Multiple source of hardfork only cause confusion and maintenance cost.

places where to look at when addressing this issue:

• test_generalstate_json.nim
• evmstate tool
• BaseVMState.fork usage, maybe in evm_call
• processTransaction