bugfix: don't shutdown graphics when showing gui message

This caused a crash in the case where if the "using the AROS ROM" message was shown, then you entered the GUI and pressed reset, triggering the message to be shown again, then pressed OK - the Amiga texture was already deleted

src/osdep/amiberry_gui.cpp

@@ -787,7 +787,6 @@ void gui_message(const char* format, ...)
 	vsprintf(msg, format, parms);
 	va_end(parms);
 
-	graphics_subshutdown();
 	ShowMessage("", msg, "", "", "Ok", "");
 }
 
@@ -1133,4 +1132,4 @@ void save_mapping_to_file(std::string mapping)
 		file_output << std::endl << mapping << std::endl;
 		file_output.close();
 	}
-}
+}