Remove unnecessary SIGINT trap for Jruby (#1961)

See #1868

The original intent of the trap was to ensure that the socket was removed
properly. It now does, so the trap is no longer necessary.

lib/puma/launcher.rb

@@ -424,12 +424,6 @@ def setup_signals
 
       begin
         Signal.trap "SIGINT" do
-          if Puma.jruby?
-            @status = :exit
-            graceful_stop
-            exit
-          end
-
           stop
         end
       rescue Exception

test/test_integration_single.rb

@@ -67,4 +67,21 @@ def test_term_not_accepts_new_connections
     @server.close unless @server.closed?
     @server = nil # prevent `#teardown` from killing already killed server
   end
+
+  def test_int_signal_with_background_thread_in_jruby
+    skip_unless :jruby
+
+    cli_server('test/rackup/hello.ru')
+    begin
+      sock = TCPSocket.new(HOST, @tcp_port)
+      sock.close
+    rescue => ex
+      fail("Port didn't open properly: #{ex.message}")
+    end
+
+    Process.kill :INT, @server.pid
+    Process.wait @server.pid
+
+    assert_raises(Errno::ECONNREFUSED) { TCPSocket.new(HOST, @tcp_port) }
+  end
 end