Revert ruby#335 (Trap TSTP to handle C-z)

This PR was an effort to address ruby#321 (ed_quoted_insert doesn't work
properly) but per the reporter it did not work correctly.

Moreover, it introduced a major regression: Shell job control stopped
working in all applications that use reline, notably IRB.

Bash and other shells send SIGTSTP in response to C-z to implement job
suspension. Handling SIGSTP opts out of this functionality. For a
line oriented terminal program this should be avoided (not to mention,
this behavior diverges from readline's)

lib/reline/line_editor.rb

@@ -239,21 +239,10 @@ def set_signal_handlers
         @old_trap.call if @old_trap.respond_to?(:call)
       end
     }
-    begin
-      @old_tstp_trap = Signal.trap('TSTP') {
-        Reline::IOGate.ungetc("\C-z".ord)
-        @old_tstp_trap.call if @old_tstp_trap.respond_to?(:call)
-      }
-    rescue ArgumentError
-    end
   end
 
   def finalize
     Signal.trap('INT', @old_trap)
-    begin
-      Signal.trap('TSTP', @old_tstp_trap)
-    rescue ArgumentError
-    end
   end
 
   def eof?