Revert "[SPARK-50849][CONNECT] Add example project to demonstrate Spark Connect Server Libraries"

### What changes were proposed in this pull request?
This PR reverts the example code provided in SPARK-50849 because it fails to compile properly.

### Why are the changes needed?
An example that fails to compile properly should not be provided to users.

The following error occurs when executing `mvn clean install -DskipTests` in the `connect-examples/server-library-example `directory:

```
[ERROR] /Users/yangjie01/SourceCode/git/spark-maven/connect-examples/server-library-example/client/src/main/scala/org/apache/connect/examples/serverlibrary/CustomTable.scala:87: overloaded method setExtension with alternatives:
  (x$1: org.sparkproject.com.google.protobuf.Any.Builder)org.apache.spark.connect.proto.Command.Builder <and>
  (x$1: org.sparkproject.com.google.protobuf.Any)org.apache.spark.connect.proto.Command.Builder
 cannot be applied to (com.google.protobuf.Any)
[ERROR] /Users/yangjie01/SourceCode/git/spark-maven/connect-examples/server-library-example/client/src/main/scala/org/apache/connect/examples/serverlibrary/CustomTable.scala:115: overloaded method setExtension with alternatives:
  (x$1: org.sparkproject.com.google.protobuf.Any.Builder)org.apache.spark.connect.proto.Relation.Builder <and>
  (x$1: org.sparkproject.com.google.protobuf.Any)org.apache.spark.connect.proto.Relation.Builder
 cannot be applied to (com.google.protobuf.Any)
[ERROR] /Users/yangjie01/SourceCode/git/spark-maven/connect-examples/server-library-example/client/src/main/scala/org/apache/connect/examples/serverlibrary/CustomTableBuilder.scala:115: overloaded method setExtension with alternatives:
  (x$1: org.sparkproject.com.google.protobuf.Any.Builder)org.apache.spark.connect.proto.Command.Builder <and>
  (x$1: org.sparkproject.com.google.protobuf.Any)org.apache.spark.connect.proto.Command.Builder
 cannot be applied to (com.google.protobuf.Any)
[ERROR] three errors found

```

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
- Pass GitHub Actions

### Was this patch authored or co-authored using generative AI tooling?
No

Closes #51261 from LuciferYang/revert-SPARK-50849.

Authored-by: yangjie01 <[email protected]>
Signed-off-by: yangjie01 <[email protected]>

Author: LuciferYang