[lldb/Plugins] Clean-up ScriptedProcess python script (NFC)

This patch removes the `my_scripted_process.py` blueprint since it's not
used anymore.

The patch also updates the base ScriptedProcess and ScriptedThread
initializers to automatically initialize convinience variables, to
access debugger from the ScriptedProcess, access the SBProcess and
ScriptedProcess object from a ScriptedThread instance.

Differential Revision: https://reviews.llvm.org/D119386



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>