Remove attempt to import deprecated ngen_routing module (NGWPC-8286)

[cmaynard-ngwpc]

The newly added code to provide better detail when importing a python module exposed a sequence in the routing code that first tries to import the ngen_routing module, which has been deprecated, then the nwm_routing, which is successful. Per direction of Nels and Bobby at OWP, removed the initial attempt to use ngen_routing and default to importing nwm_routing.

Changes

• Update LOG message wording.

Removals

• Removed attempt to import a deprecated module and associated log messages

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (link if applicable)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
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[PhilMiller]

This looks fine on its own. Any future conflict with Ian's changes to use t-route via BMI should be easy enough to resolve.