tx-generator: Core.hs: simplifications + better non-Byron support

[smelc]

Description

As part of my work on adapting tx-generator to upcoming changes to API (this is https://github.com/IntersectMBO/cardano-node/commits/smelc/adapt-to-api-removal-of-protocol-parameters-use-ledger-type-instead/), I:

1. found a place where the code could be simplified (this is the first commit)
2. found that Byron was not being supported in this place, and so removed the corresponding error case, by pushing the non-Byron constraints to callers.

Point 1. is good on its own while point 2. will make my life easier for the branch mentioned above, but it is optional (I can live with it).

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• CI passes. See note on CI. The following CI checks are required:
  • Code is linted with hlint. See .github/workflows/check-hlint.yml to get the hlint version
  • Code is formatted with stylish-haskell. See .github/workflows/stylish-haskell.yml to get the stylish-haskell version
  • Code builds on Linux, MacOS and Windows for ghc-8.10.7 and ghc-9.2.7
• Self-reviewed the diff

[NadiaYvette]

This new helper function caseByronOrShelleyBasedEra is wonderful, thanks for propagating its use onward!

[NadiaYvette]

Propagating the new helper function out to us is great & lifting the remote handling for Byron up a layer of calls is a good refactor too.