[lldb/Plugins] Clean-up Scripted Process interface requirements (NFC)

The goal of the simple patch is to clean-up the scripted process
interface by removing methods that were introduced with the interface
originally, but that were never really implemented (get_thread_with_id &
get_registers_for_thread).

This patch also changes `get_memory_region_containing_address` to have a
base implementation (that retunrs `None`), instead of forcing the user
to override it in their derived class.

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