Spanner: Remove tracing implementation from tests

[olavloite]

OpenCensus added a finalizer that checks that all spans have been closed in the latest version. Using tracer implementations in combination with tests therefore spams the logs with a lot of warnings, as much test code is deliberately only executing part of for example a transaction.

[codecov]

Codecov Report

Merging #6176 into master will decrease coverage by 0.36%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #6176 +/- ##
============================================
- Coverage 47.5% 47.14% -0.37% + Complexity 27432 25967 -1465 
============================================
Files 2523 2523 Lines 274592 269316 -5276 Branches 31401 31199 -202 ============================================
- Hits 130439 126960 -3479 + Misses 134520 133657 -863 + Partials 9633 8699 -934

Impacted Files	Coverage Δ	Complexity Δ
...m/google/cloud/vision/v1/ImageAnnotatorClient.java	58.69% <0%> (-3.31%)	15% <0%> (-4%)
...e/cloud/vision/v1p4beta1/ImageAnnotatorClient.java	58.69% <0%> (-3.31%)	15% <0%> (-4%)
...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%)
...d/webrisk/v1beta1/WebRiskServiceV1Beta1Client.java	63.41% <0%> (-2.5%)	12% <0%> (-3%)
...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%)
...oogle/cloud/language/v1/LanguageServiceClient.java	79.16% <0%> (-2.32%)	26% <0%> (-9%)
... and 640 more

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