[ruby/irb] Remove unnecessary Thread presence check

They were introduced around 20 years ago, when Thread is not yet
stabilized. So we don't need them anymore.

https://github.com/ruby/irb/commit/4c75e03b2b

lib/irb.rb

@@ -389,11 +389,7 @@ module IRB
   #
   # Will raise an Abort exception, or the given +exception+.
   def IRB.irb_abort(irb, exception = Abort)
-    if defined? Thread
-      irb.context.thread.raise exception, "abort then interrupt!"
-    else
-      raise exception, "abort then interrupt!"
-    end
+    irb.context.thread.raise exception, "abort then interrupt!"
   end
 
   class Irb

lib/irb/context.rb

@@ -32,7 +32,7 @@ module IRB
       else
         @workspace = WorkSpace.new
       end
-      @thread = Thread.current if defined? Thread
+      @thread = Thread.current
 
       # copy of default configuration
       @ap_name = IRB.conf[:AP_NAME]

lib/irb/ext/multi-irb.rb

@@ -9,7 +9,6 @@
 #
 #
 #
-fail CantShiftToMultiIrbMode unless defined?(Thread)
 
 module IRB
   class JobManager