Remove S4 classes

The `setOldClass("xts")` and `setOldClass("zoo")` calls in quantmod cause issues if other packages need to define those classes as well. For example, see https://github.com/edzer/spacetime/issues/35, and a [StackOverflow question](https://stackoverflow.com/q/44295792/271616) I answered a couple years ago.

I mentioned these to @jaryan and he would prefer to remove all the S4 classes from quantmod.

Start by locating the setOldClass("xts") and setOldClass("zoo") calls in quantmod, then inspect the package for the other S4 classes mentioned in the issue and their usages. The work is done when quantmod no longer defines those S4 classes and its existing checks or tests still pass.

Written by the indexing model from the issue text.