Fix @nullable annotations on builder methods

This fixes errors in the latest versions of AutoValue

```
google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/ConnectionSettings.java:225: error: [AutoValueBuilderSetterNullable] Setter methods always return the Builder so @nullable is not appropriate
    public abstract Builder setUseReadAPI(Boolean useReadAPI);
                            ^
```

google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/ConnectionSettings.java

@@ -221,8 +221,7 @@ Builder withDefaultValues() {
      *
      * @param useReadAPI or {@code true} for none
      */
-    @Nullable
-    public abstract Builder setUseReadAPI(Boolean useReadAPI);
+    public abstract Builder setUseReadAPI(@Nullable Boolean useReadAPI);
 
     /**
      * Sets how long to wait for the query to complete, in milliseconds, before the request times

google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/ReadClientConnectionConfiguration.java

@@ -18,7 +18,6 @@
 
 import com.google.auto.value.AutoValue;
 import java.io.Serializable;
-import javax.annotation.Nullable;
 
 /** Represents BigQueryStorage Read client connection information. */
 @AutoValue
@@ -31,21 +30,18 @@ public abstract static class Builder {
      * Sets the total row count to page row count ratio used to determine whether to us the
      * BigQueryStorage Read client to fetch result sets after the first page.
      */
-    @Nullable
     public abstract Builder setTotalToPageRowCountRatio(Long ratio);
 
     /**
      * Sets the minimum number of table rows in the query results used to determine whether to us
      * the BigQueryStorage Read client to fetch result sets after the first page.
      */
-    @Nullable
     public abstract Builder setMinResultSize(Long numRows);
 
     /**
      * Sets the maximum number of table rows allowed in buffer before streaming them to the
      * BigQueryResult.
      */
-    @Nullable
     public abstract Builder setBufferSize(Long bufferSize);
 
     /** Creates a {@code ReadClientConnectionConfiguration} object. */