Removing deprecated ServiceAccount creation in StorageImplTest.

[sduskis]

Fixes #<issue_number_goes_here> (it's a good idea to open an issue first for context and/or discussion)

[codecov]

Codecov Report

Merging #4490 into master will decrease coverage by 0.63%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #4490 +/- ##
============================================
- Coverage 49.16% 48.52% -0.64% + Complexity 21915 20739 -1176 
============================================
Files 2078 2078 Lines 207082 203004 -4078 Branches 24084 23291 -793 ============================================
- Hits 101804 98517 -3287 + Misses 97135 96437 -698 + Partials 8143 8050 -93

Impacted Files	Coverage Δ	Complexity Δ
...in/java/com/google/cloud/storage/StorageBatch.java	88% <0%> (-4%)	13% <0%> (ø)
...m/google/cloud/vision/v1/ImageAnnotatorClient.java	47.05% <0%> (-2.95%)	9% <0%> (-2%)
...e/cloud/vision/v1p3beta1/ImageAnnotatorClient.java	47.05% <0%> (-2.95%)	9% <0%> (-2%)
...e/cloud/vision/v1p2beta1/ImageAnnotatorClient.java	47.05% <0%> (-2.95%)	9% <0%> (-2%)
.../java/com/google/cloud/speech/v1/SpeechClient.java	48.57% <0%> (-2.78%)	10% <0%> (-2%)
...om/google/cloud/speech/v1p1beta1/SpeechClient.java	48.57% <0%> (-2.78%)	10% <0%> (-2%)
...ogle/cloud/texttospeech/v1/TextToSpeechClient.java	55.88% <0%> (-2.46%)	9% <0%> (-2%)
...cloud/texttospeech/v1beta1/TextToSpeechClient.java	55.88% <0%> (-2.46%)	9% <0%> (-2%)
.../src/main/java/com/google/cloud/AsyncPageImpl.java	64.28% <0%> (-2.39%)	2% <0%> (ø)
...gle/cloud/storage/testing/RemoteStorageHelper.java	55.37% <0%> (-2.36%)	8% <0%> (ø)
... and 484 more

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[igorbernstein2]

LGTM