fix(consensus): remove incomplete Linea support

[fakedev9999]

What

• Remove the built-in Linea genesis and chain-ID mapping.
• Remove Linea-specific header-validation exceptions.
• Remove the Linea end-to-end test and update supported-chain documentation.
• Verify chain ID 59144 cannot bypass standard header validation through a custom genesis.

Why

The existing path did not implement complete Linea consensus validation.

Validation

• cargo test -p rsp-primitives
• cargo test -p rsp-client-executor
• cargo check --all --all-targets
• cargo clippy --all --all-targets -- -D warnings
• cargo +nightly fmt --all -- --check
• cargo tree
• (cd bin/client && cargo tree)
• (cd book && yarn build)

Notes

Existing bincode inputs containing the removed Genesis::Linea enum variant must be regenerated.

[fakedev9999]

Superseded by #200.