[SPARK-3649] Remove GraphX custom serializers

As [reported][1] on the mailing list, GraphX throws

```
java.lang.ClassCastException: java.lang.Long cannot be cast to scala.Tuple2
        at org.apache.spark.graphx.impl.RoutingTableMessageSerializer$$anon$1$$anon$2.writeObject(Serializers.scala:39) 
        at org.apache.spark.storage.DiskBlockObjectWriter.write(BlockObjectWriter.scala:195) 
        at org.apache.spark.util.collection.ExternalSorter.spillToMergeableFile(ExternalSorter.scala:329)
```

when sort-based shuffle attempts to spill to disk. This is because GraphX defines custom serializers for shuffling pair RDDs that assume Spark will always serialize the entire pair object rather than breaking it up into its components. However, the spill code path in sort-based shuffle [violates this assumption][2].

GraphX uses the custom serializers to compress vertex ID keys using variable-length integer encoding. However, since the serializer can no longer rely on the key and value being serialized and deserialized together, performing such encoding would require writing a tag byte.

Instead, this PR simply removes the custom serializers. This causes a 10% slowdown for PageRank (494 s to 543 s, PageRank, 3 trials, 10 iterations per trial, uk-2007-05 graph, 16 r3.2xlarge nodes).

[1]: http://apache-spark-user-list.1001560.n3.nabble.com/java-lang-ClassCastException-java-lang-Long-cannot-be-cast-to-scala-Tuple2-td13926.html#a14501
[2]: https://github.com/apache/spark/blob/f9d6220c792b779be385f3022d146911a22c2130/core/src/main/scala/org/apache/spark/util/collection/ExternalSorter.scala#L329