Remove builder method tests from BigtableIO, it uses AutoValue now

[iemejia]

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[iemejia]

R: @dhalperi

[dhalperi]

Why is this code not still providing useful coverage?

Specifically, it tests that the code that we wrote around the AutoValue builder is correct.

[dhalperi]

(e.g., https://github.com/apache/beam/blob/master/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigtable/BigtableIO.java#L274)

[iemejia]

You have a point, I did this PR since I have seen some discussion around removing 'obvious' test cases in the moment we were reviewing the PTransform/IO guides. If this is not the expected behavior I will close then this PR.