Revert "Workaround for mmtk/mmtk-core#654 (#22)"

This reverts commit de7fb26.

The upstream fixed the printing issue in:
mmtk/mmtk-core#655

Author: wks

bootstraptest/runner.rb

@@ -469,10 +469,7 @@ def get_result_string(opt = '', **argh)
         kw = self.err ? {err: self.err} : {}
         out = IO.popen("#{BT.ruby} -W0 #{opt} #{filename}", **kw)
         pid = out.pid
-        result = out.read.tap{ Process.waitpid(pid); out.close }
-        # https://github.com/mmtk/mmtk-core/issues/654
-        result.gsub! /Warning: User attempted a collection request, but it is not supported in NoGC. The request is ignored.\n/, ''
-        result
+        out.read.tap{ Process.waitpid(pid); out.close }
       ensure
         raise Interrupt if $? and $?.signaled? && $?.termsig == Signal.list["INT"]