[SPARK-48975][PROTOBUF] Remove unnecessary ScalaReflectionLock defi…

…nition from `protobuf`

### What changes were proposed in this pull request?
This PR removes the unused object definition `ScalaReflectionLock` from the `protobuf` module. `ScalaReflectionLock` is a definition at the access scope of `protobuf` package, which was defined in SPARK-40654 | apache#37972 and become unused in SPARK-41639 | apache#39147.

### Why are the changes needed?
Clean up unused definitions.

### 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 apache#47459 from LuciferYang/remove-ScalaReflectionLock.

Authored-by: yangjie01 <yangjie01@baidu.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

connector/protobuf/src/main/scala/org/apache/spark/sql/protobuf/package.scala

@@ -17,5 +17,4 @@
 package org.apache.spark.sql
 
 package object protobuf {
-  protected[protobuf] object ScalaReflectionLock
 }