Revert "alias assert_raise_message for compatibility with test-unit"

This reverts commit 43015275b9.

`assert_raise_message` in test-unit is different from
`assert_raise_with_message`.  It checks the exception message
only, but not the exception class,

tool/lib/test/unit/core_assertions.rb

@@ -275,7 +275,6 @@ eom
         end
         ex
       end
-      alias assert_raise_message assert_raise_with_message
 
       def assert_warning(pat, msg = nil)
         result = nil