Only recognise Pod for .pod files (#3863)

We uncomplicate matters by removing ".pod" from the Perl definition
entirely.

test/test_heuristics.rb

@@ -1,6 +1,6 @@
 require_relative "./helper"
 
-class TestHeuristcs < Minitest::Test
+class TestHeuristics < Minitest::Test
   include Linguist
 
   def fixture(name)
@@ -237,14 +237,6 @@ class TestHeuristcs < Minitest::Test
     })
   end
 
-  # Candidate languages = ["Pod", "Perl"]
-  def test_pod_by_heuristics
-    assert_heuristics({
-      "Perl" => all_fixtures("Perl", "*.pod"),
-      "Pod" => all_fixtures("Pod", "*.pod")
-    })
-  end
-
   # Candidate languages = ["IDL", "Prolog", "QMake", "INI"]
   def test_pro_by_heuristics
     assert_heuristics({