[SPARK-45364][INFRA][BUILD] Clean up the unnecessary Scala 2.12 logical in SparkBuild

### What changes were proposed in this pull request?
The pr aims to clean up the unnecessary Scala 2.12 logical in SparkBuild.

### Why are the changes needed?
Spark 4.0 no longer supports Scala 2.12.

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

### How was this patch tested?
Pass GA.

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

Closes #43158 from panbingkun/SPARK-45364.

Authored-by: panbingkun <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

Author: panbingkun

project/SparkBuild.scala

@@ -352,10 +352,7 @@ object SparkBuild extends PomBuild {
         "org.apache.spark.util.collection"
       ).mkString(":"),
       "-doc-title", "Spark " + version.value.replaceAll("-SNAPSHOT", "") + " ScalaDoc"
-    ) ++ {
-      // Do not attempt to scaladoc javadoc comments under 2.12 since it can't handle inner classes
-      if (scalaBinaryVersion.value == "2.12") Seq("-no-java-comments") else Seq.empty
-    },
+    ),
 
     // disable Mima check for all modules,
     // to be enabled in specific ones that have previous artifacts