Provide only one way to run simulation
Created by: JeanRochCoulon
README.md how to run a simulation by executing Makefile commands, for instance 'make verilate'. This way does not work because additional variables need to be setup before executing these commands, for instance TARGET=cv64a6_imafdc_sv39. Many GitHub issues request support as #1489 (closed). This PR removes the getting start related to these commands to keep the way which is support that is to say using cva6.py command.
@MikeOpenHWGroup
@zarubaf
can you approve ?