chore(context): remove unreachable EIP-7873 derive error

[vh0rvath]

Remove the unreachable MissingTargetForEip7873 path in TxEnv and align docs, so derive/build error modeling matches executable transaction-type logic.

[codspeed-hq]

Merging this PR will not alter performance

✅ 176 untouched benchmarks
⏩ 1 skipped benchmark¹

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_{Comparing vh0rvath:remove-unreachable-eip7873-derive-error (6255c42) with main (e776bc5)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩