[LLDB][Python] remove ArgInfo::count

Summary:
This patch updates the last user of ArgInfo::count and deletes
it.   I also delete `GetNumInitArguments()` and `GetInitArgInfo()`.
Classess are callables and `GetArgInfo()` should work on them.

On python 3 it already works, of course. `inspect` is good.

On python 2 we have to add yet another special case.   But hey if
python 2 wasn't crufty we wouln't need python 3.

I also delete `is_bound_method` becuase it is unused.

This path is tested in `TestStepScripted.py`

Reviewers: labath, mgorny, JDevlieghere

Reviewed By: labath, JDevlieghere

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D69742