[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

lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.cpp

@@ -807,8 +807,10 @@ ScriptInterpreterPythonImpl::GetMaxPositionalArgumentsForCallable(
         llvm::inconvertibleErrorCode(),
         "can't find callable: %s", callable_name.str().c_str());
   }
-  PythonCallable::ArgInfo arg_info = pfunc.GetNumArguments();
-  return arg_info.max_positional_args;
+  llvm::Expected<PythonCallable::ArgInfo> arg_info = pfunc.GetArgInfo();
+  if (!arg_info)
+    return arg_info.takeError();
+  return arg_info.get().max_positional_args;
 }
 
 static std::string GenerateUniqueName(const char *base_name_wanted,