Remove the :mjit feature in specs

* Use `guard_not -> { defined?(RubyVM::MJIT) && RubyVM::MJIT.enabled? } do` instead
* There is only one spec needing it currently.
* Features are being deprecated in MSpec.

core/io/shared/write.rb

@@ -85,7 +85,11 @@
       @r.read.should == "foo"
     end
 
-    without_feature :mjit do # [ruby-core:90895] MJIT worker may leave fd open in a forked child. TODO: consider acquiring GVL from MJIT worker.
+    # [ruby-core:90895] MJIT worker may leave fd open in a forked child.
+    # For instance, MJIT creates a worker before @r.close with fork(), @r.close happens,
+    # and the MJIT worker keeps the pipe open until the worker execve().
+    # TODO: consider acquiring GVL from MJIT worker.
+    guard_not -> { defined?(RubyVM::MJIT) && RubyVM::MJIT.enabled? } do
       it "raises Errno::EPIPE if the read end is closed and does not die from SIGPIPE" do
         @r.close
         -> { @w.send(@method, "foo") }.should raise_error(Errno::EPIPE, /Broken pipe/)

default.mspec

@@ -46,7 +46,4 @@ class MSpecScript
     /\wSpecs?$/,
     /^CS_CONST/,
   ]
-
-  # Enable features
-  MSpec.enable_feature :mjit if defined?(RubyVM::MJIT) && RubyVM::MJIT.enabled?
 end