Remove check for (now optional) ENV-variable for GCP Authentication

[lassetyr]

Google now supports/recommends defining pubsub-authentication through "workload identity" (https://cloud.google.com/kubernetes-engine/docs/concepts/workload-identity).

This PR removes the check for the - now optional - ENV variable GOOGLE_APPLICATION_CREDENTIALS, and thus assumes that config is correct when starting up.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.01 🎉

Comparison is base (703d25c) 63.86% compared to head (0f699c4) 63.88%.

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #4966      +/-   ##
=============================================
+ Coverage      63.86%   63.88%   +0.01%     
- Complexity     13472    13481       +9     
=============================================
  Files           1674     1674              
  Lines          66100    66107       +7     
  Branches        7150     7145       -5     
=============================================
+ Hits           42216    42233      +17     
+ Misses         21498    21491       -7     
+ Partials        2386     2383       -3     

Impacted Files	Coverage Δ
...er/ext/siri/updater/SiriETGooglePubsubUpdater.java	0.00% <0.00%> (ø)

... and 6 files with indirect coverage changes

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