Drop sets preventing cycles (cause there are none anymore)

src/compiler/crystal/semantic/abstract_def_checker.cr

@@ -24,7 +24,6 @@
 # ```
 class Crystal::AbstractDefChecker
   def initialize(@program : Program)
-    @all_checked = Set(Type).new
   end
 
   def run
@@ -41,12 +40,6 @@ class Crystal::AbstractDefChecker
   end
 
   def check_single(type)
-    if @all_checked.includes?(type)
-      ::raise "AbstractDefChecker duplicate: #{type} (#{type.class})"
-      return
-    end
-    @all_checked << type
-
     if type.abstract? || type.module?
       type.defs.try &.each_value do |defs_with_metadata|
         defs_with_metadata.each do |def_with_metadata|

src/compiler/crystal/semantic/recursive_struct_checker.cr

@@ -14,10 +14,8 @@
 # Because the type of `Test.@test` would be: `Test | Nil`.
 class Crystal::RecursiveStructChecker
   @program : Program
-  @all_checked : Set(Type)
 
   def initialize(@program)
-    @all_checked = Set(Type).new
   end
 
   def run
@@ -34,12 +32,6 @@ class Crystal::RecursiveStructChecker
   end
 
   def check_single(type)
-    has_not_been_checked = @all_checked.add?(type)
-    unless has_not_been_checked
-      raise "RecursiveStructChecker duplicate: #{type} (#{type.class})"
-      return
-    end
-
     if struct?(type)
       target = type
       checked = Set(Type).new

src/compiler/crystal/semantic/type_declaration_processor.cr

@@ -621,17 +621,10 @@ struct Crystal::TypeDeclarationProcessor
   end
 
   private def remove_duplicate_instance_vars_declarations
-    # All the types that we checked for duplicate variables
-    duplicates_checked = Set(Type).new
-    remove_duplicate_instance_vars_declarations(@program, duplicates_checked)
+    remove_duplicate_instance_vars_declarations(@program)
   end
 
-  private def remove_duplicate_instance_vars_declarations(type : Type, duplicates_checked : Set(Type))
-    unless duplicates_checked.add?(type)
-      raise "remove_duplicate_instance_vars_declarations duplicate: #{type} (#{type.class})"
-      return
-    end
-
+  private def remove_duplicate_instance_vars_declarations(type : Type)
     # If a class has an instance variable that already exists in a superclass, remove it.
     # Ideally we should process instance variables in a top-down fashion, but it's tricky
     # with modules and multiple-inheritance. Removing duplicates at the end is maybe
@@ -653,7 +646,7 @@ struct Crystal::TypeDeclarationProcessor
     end
 
     type.types?.try &.each_value do |nested_type|
-      remove_duplicate_instance_vars_declarations(nested_type, duplicates_checked)
+      remove_duplicate_instance_vars_declarations(nested_type)
     end
   end