Revert "Use MiniTest in Ruby 1.8 if it is available."

This reverts commit c3e186ec8d.

Reason: too many incompatibilities make a lot of Rails tests fail in 1.9.
Might need more work to get this done properly in 3-2 if necessary.

activesupport/CHANGELOG.md

@@ -2,11 +2,6 @@
 
 *   Fix mocha v0.13.0 compatibility. *James Mead*
 
-*   `ActiveSupport::TestCase` now inherits from `Minitest::Unit::TestCase` in Ruby 1.8
-    if available, otherwise it inherits from the default `Test::Unit::TestCase`.
-
-    *James Mead*
-
 *   `#as_json` isolates options when encoding a hash. [Backport #8185]
     Fix #8182
 

activesupport/lib/active_support/test_case.rb

@@ -9,9 +9,7 @@ require 'active_support/testing/mochaing'
 require 'active_support/core_ext/kernel/reporting'
 
 module ActiveSupport
-  test_library = defined?(MiniTest) ? ::MiniTest : ::Test
-
-  class TestCase < test_library::Unit::TestCase
+  class TestCase < ::Test::Unit::TestCase
     if defined? MiniTest
       Assertion = MiniTest::Assertion
       alias_method :method_name, :name if method_defined? :name