Remove Unicode test from series exercise to be consistent with instructions (#1567)

Author: codingthat

exercises/practice/series/test/string_series_test.exs

@@ -65,12 +65,6 @@ defmodule StringSeriesTest do
     assert StringSeries.slices("01234", 5) == ["01234"]
   end
 
-  @tag :pending
-  test "Unicode characters count as a single character" do
-    assert StringSeries.slices("José", 1) == ["J", "o", "s", "é"]
-    assert StringSeries.slices("José", 2) == ["Jo", "os", "sé"]
-  end
-
   @tag :pending
   test "slices with size longer than string return empty list" do
     assert StringSeries.slices("01234", 6) == []