Description

core: LoadChainConfig return the predefined config for built-in networks firstly

Rationale

1. for the built-in networks, the predefined config is the standard
   and they should not be configured in the config.toml
   so return the predefined config
2. furtherly, for local test or private network, can use geth init to upgrade the chain config,
   so we just stop supporting read chain config from the config.toml.
   by doing this, logic about chain config can be simplified.

Example

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Changes

Notable changes:

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