Revert "[SPARK-3812] [BUILD] Adapt maven build to publish effective pom."

This reverts commit c5882c66.

I am reverting this becuase it appears to cause the maven tests
to hang.

pom.xml

@@ -248,17 +248,7 @@
       </snapshots>
     </pluginRepository>
   </pluginRepositories>
-  <!-- 
-       This is a dummy dependency that is used along with the shading plug-in
-       to create effective poms on publishing (see SPARK-3812).
-  -->
-  <dependencies>
-    <dependency>
-      <groupId>org.spark-project.spark</groupId>
-      <artifactId>unused</artifactId>
-      <version>1.0.0</version>
-    </dependency>
-  </dependencies>
+
   <dependencyManagement>
     <dependencies>
       <dependency>
@@ -1002,27 +992,6 @@
     </pluginManagement>
 
     <plugins>
-      <!-- The shade plug-in is used here to create effective pom's (see SPARK-3812). -->
-      <plugin>
-        <groupId>org.apache.maven.plugins</groupId>
-        <artifactId>maven-shade-plugin</artifactId>
-        <executions>
-          <execution>
-            <phase>package</phase>
-            <goals>
-              <goal>shade</goal>
-            </goals>
-            <configuration>
-              <shadedArtifactAttached>false</shadedArtifactAttached>
-              <artifactSet>
-                <includes>
-                  <include>org.spark-project.spark:unused</include>
-                </includes>
-              </artifactSet>
-            </configuration>
-          </execution>
-        </executions>
-      </plugin>
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-enforcer-plugin</artifactId>