[MINOR][CORE][TEST] Update obsolete `takeSample` test case.

## What changes were proposed in this pull request?

This PR fixes some obsolete comments and assertion in `takeSample` testcase of `RDDSuite.scala`.

## How was this patch tested?

This fixes the testcase only.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #13260 from dongjoon-hyun/SPARK-15481.

core/src/test/scala/org/apache/spark/rdd/RDDSuite.scala

@@ -678,27 +678,26 @@ class RDDSuite extends SparkFunSuite with SharedSparkContext {
     }
     {
       val sample = data.takeSample(withReplacement = true, num = 20)
-      assert(sample.size === 20)        // Got exactly 100 elements
-      assert(sample.toSet.size <= 20, "sampling with replacement returned all distinct elements")
+      assert(sample.size === 20)        // Got exactly 20 elements
       assert(sample.forall(x => 1 <= x && x <= n), s"elements not in [1, $n]")
     }
     {
       val sample = data.takeSample(withReplacement = true, num = n)
-      assert(sample.size === n)        // Got exactly 100 elements
-      // Chance of getting all distinct elements is astronomically low, so test we got < 100
+      assert(sample.size === n)        // Got exactly n elements
+      // Chance of getting all distinct elements is astronomically low, so test we got < n
       assert(sample.toSet.size < n, "sampling with replacement returned all distinct elements")
       assert(sample.forall(x => 1 <= x && x <= n), s"elements not in [1, $n]")
     }
     for (seed <- 1 to 5) {
       val sample = data.takeSample(withReplacement = true, n, seed)
-      assert(sample.size === n)        // Got exactly 100 elements
-      // Chance of getting all distinct elements is astronomically low, so test we got < 100
+      assert(sample.size === n)        // Got exactly n elements
+      // Chance of getting all distinct elements is astronomically low, so test we got < n
       assert(sample.toSet.size < n, "sampling with replacement returned all distinct elements")
     }
     for (seed <- 1 to 5) {
       val sample = data.takeSample(withReplacement = true, 2 * n, seed)
-      assert(sample.size === 2 * n)        // Got exactly 200 elements
-      // Chance of getting all distinct elements is still quite low, so test we got < 100
+      assert(sample.size === 2 * n)        // Got exactly 2 * n elements
+      // Chance of getting all distinct elements is still quite low, so test we got < n
       assert(sample.toSet.size < n, "sampling with replacement returned all distinct elements")
     }
   }