Remove pathway id from REST API

[leonardehrenfried]

Summary

The REST API used to expose the ID of the first pathway on a street leg. This is a strange piece of the API that thankfully didn't make it into the GraphQL API.

Because of this, I'm removing it from the API and also do the associated internal clean up so that we don't needlessly pass the ID around.

Lastly, I renamed a few properties in WalkStep so they better reflect their meaning.

@miles-grant-ibigroup has confirmed that IBI is not using this field.

[codecov]

Codecov Report

Patch coverage: 86.95% and project coverage change: +0.12% 🎉

Comparison is base (1baa283) 66.08% compared to head (56bd810) 66.20%.
Report is 76 commits behind head on dev-2.x.

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #5303      +/-   ##
=============================================
+ Coverage      66.08%   66.20%   +0.12%     
- Complexity     14942    15140     +198     
=============================================
  Files           1780     1787       +7     
  Lines          68969    69205     +236     
  Branches        7287     7346      +59     
=============================================
+ Hits           45576    45816     +240     
- Misses         20905    20909       +4     
+ Partials        2488     2480       -8     

Files Changed	Coverage Δ
...builder/module/AddTransitModelEntitiesToGraph.java	59.50% <ø> (+0.22%)	⬆️
.../main/java/org/opentripplanner/model/plan/Leg.java	77.92% <ø> (-0.29%)	⬇️
...java/org/opentripplanner/model/plan/StreetLeg.java	70.96% <ø> (+0.19%)	⬆️
...g/opentripplanner/model/plan/StreetLegBuilder.java	100.00% <ø> (ø)
.../algorithm/mapping/GraphPathToItineraryMapper.java	88.20% <ø> (+0.70%)	⬆️
...tripplanner/street/model/edge/ElevatorHopEdge.java	63.04% <ø> (-1.54%)	⬇️
...ext/transmodelapi/model/plan/PathGuidanceType.java	91.11% <50.00%> (ø)
.../java/org/opentripplanner/model/plan/WalkStep.java	78.94% <66.66%> (ø)
...ing/algorithm/mapping/StatesToWalkStepsMapper.java	82.93% <85.71%> (ø)
...nner/ext/gtfsgraphqlapi/datafetchers/stepImpl.java	77.27% <100.00%> (+1.08%)	⬆️
... and 4 more

... and 50 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.