[analyzer][RetainCount] Remove the CheckOSObject option

As per http://lists.llvm.org/pipermail/cfe-dev/2019-August/063215.html, lets get rid of this option.

It presents 2 issues that have bugged me for years now:

* OSObject is NOT a boolean option. It in fact has 3 states:
  * osx.OSObjectRetainCount is enabled but OSObject it set to false: RetainCount
    regards the option as disabled.
  * sx.OSObjectRetainCount is enabled and OSObject it set to true: RetainCount
    regards the option as enabled.
  * osx.OSObjectRetainCount is disabled: RetainCount regards the option as
    disabled.
* The hack involves directly modifying AnalyzerOptions::ConfigTable, which
  shouldn't even be public in the first place.

This still isn't really ideal, because it would be better to preserve the option
and remove the checker (we want visible checkers to be associated with
diagnostics, and hidden options like this one to be associated with changing how
the modeling is done), but backwards compatibility is an issue.

Differential Revision: https://reviews.llvm.org/D78097