Remove ENV RUBY_MN_THREADS=1 (#10090)

Reported in https://github.com/puma/puma/issues/3715#issuecomment-3245560096. Setting this value tanks the performance of Puma 7.0.0.pre1. Independently, there are other reports that there are some bugs with this feature

- https://bugs.ruby-lang.org/issues/20638
- https://bugs.ruby-lang.org/issues/19996
- https://github.com/puma/puma/issues/3715#issuecomment-3247114128

I also work at Heroku where I see many many (cannot say a number, but it's a lot) of production Ruby apps, and I don't think I've ever seen one use this in production (not saying it doesn't happen, but that it's not the default).

frameworks/Ruby/rack/rack.dockerfile

@@ -1,7 +1,6 @@
 FROM ruby:3.5-rc
 
 ENV RUBY_YJIT_ENABLE=1
-ENV RUBY_MN_THREADS=1
 ENV RUBY_THREAD_TIMESLICE=10
 
 # Use Jemalloc