Remove 2.0 workarounds

Author: pirj

lib/rspec/mocks/method_double.rb

@@ -208,7 +208,7 @@ def raise_method_not_stubbed_error
 
       private
 
-      # In Ruby 2.0.0 and above prepend will alter the method lookup chain.
+      # Prepend alters the method lookup chain.
       # We use an object's singleton class to define method doubles upon,
       # however if the object has had its singleton class (as opposed to
       # its actual class) prepended too then the the method lookup chain
@@ -218,7 +218,6 @@ def raise_method_not_stubbed_error
       # This code works around that by providing a mock definition target
       # that is either the singleton class, or if necessary, a prepended module
       # of our own.
-      #
 
       # We subclass `Module` in order to be able to easily detect our prepended module.
       RSpecPrependedModule = Class.new(Module)

lib/rspec/mocks/proxy.rb

@@ -233,11 +233,7 @@ def visibility_for(_method_name)
 
       def self.prepended_modules_of(klass)
         ancestors = klass.ancestors
-
-        # `|| 0` is necessary for Ruby 2.0, where the singleton class
-        # is only in the ancestor list when there are prepended modules.
-        singleton_index = ancestors.index(klass) || 0
-
+        singleton_index = ancestors.index(klass)
         ancestors[0, singleton_index]
       end