drop unused stackdriver unit test workflow

[dprotaso]

Fixes: #2173

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[codecov]

Codecov Report

Merging #2192 (b68d7ee) into main (00fa154) will decrease coverage by 1.45%.
The diff coverage is 70.73%.

@@            Coverage Diff             @@
##             main    #2192      +/-   ##
==========================================
- Coverage   67.58%   66.13%   -1.46%     
==========================================
  Files         215      221       +6     
  Lines        9098     9257     +159     
==========================================
- Hits         6149     6122      -27     
- Misses       2675     2861     +186     
  Partials      274      274              

Impacted Files	Coverage Δ
.../injection-gen/generators/reconciler_controller.go	0.00% <ø> (ø)
leaderelection/config.go	77.27% <ø> (ø)
logging/warning_handler.go	0.00% <0.00%> (ø)
metrics/config_observability.go	92.00% <ø> (ø)
source/source_stats_reporter.go	84.90% <ø> (ø)
test/cleanup.go	0.00% <0.00%> (ø)
test/clients.go	0.00% <ø> (ø)
test/request.go	0.00% <0.00%> (ø)
test/spoof/error_checks.go	76.92% <0.00%> (-6.42%)	⬇️
tracing/opencensus.go	46.26% <ø> (+3.21%)	⬆️
... and 28 more

---

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[dprotaso]

/assign @skonto

[skonto]

Looks good, let's deal with the codecov issue seperately.

[skonto]

/lgtm