Drop "spark." requirement.

After thinking about it a bit more, it can lead to confusing behavior.
Since substitution only applies to configs declared internally by Spark,
this avoids random user configs being affected already, so no need for
another check to disable that.

Author: Marcelo Vanzin

core/src/main/scala/org/apache/spark/internal/config/ConfigEntry.scala

@@ -171,14 +171,10 @@ private object ConfigEntry {
       val replacement = prefix match {
         case null =>
           require(!usedRefs.contains(name), s"Circular reference in $value: $name")
-          if (name.startsWith("spark.")) {
-            Option(findEntry(name))
-              .flatMap(_.readAndExpand(conf, getenv, usedRefs = usedRefs + name))
-              .orElse(Option(conf.get(name)))
-              .orElse(defaultValueString(name))
-          } else {
-            None
-          }
+          Option(findEntry(name))
+            .flatMap(_.readAndExpand(conf, getenv, usedRefs = usedRefs + name))
+            .orElse(Option(conf.get(name)))
+            .orElse(defaultValueString(name))
         case "system" => sys.props.get(name)
         case "env" => Option(getenv(name))
         case _ => None