Remove an unused method with its outdated comment (#4212)

Minor cleanup in ShimLoader.scala
- unused method `setSparkShimProviderClass` that used to power the shim override option
- unnecessary self-reference in `newExplainPlan`

Fixes #4213 

Signed-off-by: Gera Shegalov <gera@apache.org>

sql-plugin/src/main/scala/com/nvidia/spark/rapids/ShimLoader.scala

@@ -355,15 +355,6 @@ object ShimLoader extends Logging {
     SPARK_BUILD_DATE
   )
 
-  // TODO broken right now, check if this can be supported with parallel worlds
-  // it implies the prerequisite of having such a class in the conventional root jar entry
-  // - or the necessity of an additional parameter for specifying the shim subdirectory
-  // - or enforcing the convention the class file parent directory is the shimId that is also
-  //   a top entry e.g. /spark301/com/nvidia/test/shim/spark301/Spark301Shims.class
-  def setSparkShimProviderClass(classname: String): Unit = {
-    shimProviderClass = classname
-  }
-
   def loadClass(className: String): Class[_] = {
     val loader = getShimClassLoader()
     logDebug(s"Loading $className using $loader with the parent loader ${loader.getParent}")
@@ -428,6 +419,6 @@ object ShimLoader extends Logging {
   }
 
   def newExplainPlan(): ExplainPlanBase = {
-    ShimLoader.newInstanceOf[ExplainPlanBase]("com.nvidia.spark.rapids.ExplainPlanImpl")
+    newInstanceOf[ExplainPlanBase]("com.nvidia.spark.rapids.ExplainPlanImpl")
   }
 }