[lldb/Process/Windows] Attempting to kill exited/detached process in not an error

The lldb test-suite on Windows reports a 'CLEANUP ERROR' when attempting to kill
an exited/detached process. This change makes ProcessWindows consistent with
the other processes which only log the error. After this change a number of
'CLEANUP ERROR' messages are now removed.

Differential Revision: https://reviews.llvm.org/D84957