[GR-35405] Do not set Xmx for specs & MRI tests

* It's too hard to predict a correct value and it can result in OOM
  on some platforms, or excessive GC which can cause timeouts.
  The default heap is used instead.

Author: eregon

tool/jt.rb

@@ -1216,7 +1216,7 @@ def test(*args)
 
     truffle_args = []
     if truffleruby?
-      vm_args, ruby_args, _parsed_options = ruby_options({}, %w[--vm.Xmx2G --reveal --experimental-options --testing-rubygems])
+      vm_args, ruby_args, _parsed_options = ruby_options({}, %w[--reveal --experimental-options --testing-rubygems])
       truffle_args = vm_args + ruby_args
     end
 
@@ -1590,7 +1590,6 @@ def mspec(*args)
     end
 
 
-    vm_args << (truffleruby_native? ? '--vm.Xmx3G' : '--vm.Xmx2G')
     vm_args << '--polyglot' if truffleruby_jvm?
     # Until pattern matching is complete, we enable it in specs but not globally
     vm_args << '--experimental-options' << '--pattern-matching'