Remove dummy MatchData type

Also fix for undefined MatchData type in Matcher types.

src/spectator/matchers/dummy_match_data.cr

@@ -1,30 +0,0 @@
-require "./match_data"
-
-module Spectator::Matchers
-  # Match data that does nothing useful.
-  # This is a workaround for a compilation issue - hopefully temporary.
-  # This might be the same issue that `DummyExample` solves.
-  # Without this, the Crystal compiler complains:
-  # ```text
-  # undefined method 'Spectator::Matchers::MatchData#message()'
-  # ```
-  # The method is clearly defined by `MatchData`, just abstract.
-  # Additionally, there are implementations of `MatchData` -
-  # see `NilMatcher::MatchData`.
-  private struct DummyMatchData < MatchData
-    # Dummy values.
-    def values
-      {expected: nil, actual: nil}
-    end
-
-    # Dummy message.
-    def message
-      "DUMMY"
-    end
-
-    # Dummy message.
-    def negated_message
-      "DUMMY"
-    end
-  end
-end

src/spectator/matchers/matcher.cr

@@ -1,3 +1,5 @@
+require "./match_data"
+
 module Spectator::Matchers
   # Common base class for all expectation conditions.
   # A matcher looks at something produced by the SUT