[Task] Update test to remove manual V1/V2 switch when AUTO is supported.

[TimothyW553]

#5833

As of Jan 21, 2026 we need to manually switch between V2 strict and V1 to perform writes and V2 reads. Once the AUTO mode for V1 fallback is merged, then we can modify this test to make sure that it uses auto.

Also to remove necessary code: UCDeltaStreamingTest::restoreV2Mode

To also remove constants in UCDeltaTableIntegrationBaseTest.java

  protected static final String V2_ENABLE_MODE_KEY = "spark.databricks.delta.v2.enableMode";
  protected static final String V2_ENABLE_MODE_STRICT = "STRICT";
  protected static final String V2_ENABLE_MODE_NONE = "NONE";