remove multi-argument form of PythonObject::Reset()

Summary:
With this patch, only the no-argument form of `Reset()` remains in
PythonDataObjects.   It also deletes PythonExceptionState in favor of
PythonException, because the only call-site of PythonExceptionState was
also using Reset, so I cleaned up both while I was there.

Reviewers: JDevlieghere, clayborg, labath, jingham

Reviewed By: labath

Subscribers: mgorny, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D69214

llvm-svn: 375475