nucleotide-count: Remove special treatment of uracil

For the related discussion see https://github.com/exercism/xpython/pull/98.

nucleotide-count/example.java

   }
 
   private static boolean isCountable(char base) {
-    final String COUNTABLE_NUCLEOTIDES = "ACGTU";
+    final String COUNTABLE_NUCLEOTIDES = "ACGT";
     return COUNTABLE_NUCLEOTIDES.indexOf(base) == -1;
   }
 

nucleotide-count/src/test/java/NucleotideTest.java

   }
 
   @Test
-  public void testDnaHasNoUracil() {
-    DNA dna = new DNA("CGATTGGG");
-    assertThat(dna.count('U')).isEqualTo(0);
-  }
-
-  @Test
-  public void testDnaCountsDoNotChangeAfterCountingUracil() {
+  public void testDnaCountsDoNotChangeAfterCountingAdenosine() {
     DNA dna = new DNA("GATTACA");
-    dna.count('U');
+    dna.count('A');
     assertThat(dna.nucleotideCounts()).hasSize(4).contains(
         entry('A', 3),
         entry('C', 1),