Allow underscores in GTFS feed IDs

[leonardehrenfried]

Summary

Right now it's not allowed to have an underscore in a GTFS feed ID - the underscore is stripped out. This seems like a relict from another time where this ID was used differently and I cannot see a reason why we shouldn't allow the underscore.

This PR fixes that and also does a bit of gentle clear out of related areas in the code.

Unit tests

Added.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.06 🎉

Comparison is base (14087c8) 65.36% compared to head (c3c57e4) 65.42%.

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #5191      +/-   ##
=============================================
+ Coverage      65.36%   65.42%   +0.06%     
- Complexity     14456    14478      +22     
=============================================
  Files           1754     1755       +1     
  Lines          67925    67984      +59     
  Branches        7231     7231              
=============================================
+ Hits           44396    44477      +81     
+ Misses         21059    21041      -18     
+ Partials        2470     2466       -4     

Impacted Files	Coverage Δ
.../opentripplanner/gtfs/graphbuilder/GtfsBundle.java	70.90% <ø> (ø)
...pplanner/gtfs/graphbuilder/GtfsFeedParameters.java	100.00% <ø> (ø)
...entripplanner/graph_builder/module/GtfsFeedId.java	92.30% <100.00%> (ø)

... and 15 files with indirect coverage changes

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