Remove unnecessary sourceType attribute from GTFS-RT configuration

[leonardehrenfried]

Summary

In #4517 the field configType for GTFS-RT updaters was changed leading to users having to update their config files. This led me to investigate if the field is actually necessary and it turns out that it isn't!

Removing this field resolves a couple of long standing TODOs and will make configuration easier.

Documentation

Updated.

[codecov-commenter]

Codecov Report

Base: 59.70% // Head: 59.69% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (d003da2) compared to base (14a9659).
Patch coverage: 18.18% of modified lines in pull request are covered.

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #4525      +/-   ##
=============================================
- Coverage      59.70%   59.69%   -0.01%     
  Complexity     11881    11881              
=============================================
  Files           1556     1556              
  Lines          62390    62371      -19     
  Branches        6966     6966              
=============================================
- Hits           37252    37235      -17     
+ Misses         22988    22987       -1     
+ Partials        2150     2149       -1     

Impacted Files	Coverage Δ
...pentripplanner/ext/siri/updater/SiriETUpdater.java	0.00% <ø> (ø)
...pentripplanner/ext/siri/updater/SiriVMUpdater.java	0.00% <ø> (ø)
.../config/updaters/PollingStoptimeUpdaterConfig.java	63.15% <0.00%> (-10.76%)	⬇️
...va/org/opentripplanner/updater/DataSourceType.java	100.00% <ø> (ø)
...entripplanner/updater/trip/PollingTripUpdater.java	0.00% <0.00%> (ø)
...ner/updater/trip/PollingTripUpdaterParameters.java	47.82% <ø> (+1.67%)	⬆️
...ehicle_position/PollingVehiclePositionUpdater.java	0.00% <ø> (ø)
...config/updaters/VehiclePositionsUpdaterConfig.java	88.88% <100.00%> (+4.67%)	⬆️

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