Stop overwriting forking config in extendConfig

[cgewecke]

#891

The existing logic for extendConfig tries to clone the user's config and apply it to the resolved HH config after updating gas values, accidentally overwriting some necessary default fields in the forking object, which looks like this:

 forking: {
    url: 'https://opt-mainnet.g.alchemy.com/v2/....',
    enabled: true,
    httpHeaders: {},
    blockNumber: 123190000
  },

This isn't necessary, the config object is already seeded with user's values when extendConfig executes.