Revert "Don't leak the debug sessions"

The reverted commit made the debugger plugin a debug session's parent in
order to fix test_midbus memory leaks. However, DebugController is
responsible for destroying debug sessions, not the debugger plugin. This
unique ownership works correctly in KDevelop. See my older fixes:
89ffddc7 and
37ff587e. Potentially destroying a debug
session earlier, before the last safe moment in ~DebugController(), may
unnecessarily kill its debugger process.

With the debugger plugin as a debug session's parent, disabling the GDB
plugin while debugging an application breaks debugging and can even
cause a crash if MIDebugJob::doKill() accesses a dangling raw pointer
m_session. With the session-parenting commit reverted, miraculously,
disabling the GDB plugin does not affect debugging, which continues
working properly as though nothing has changed; starting a new debug
session fails as expected when the GDB plugin is disabled.

The reverted commit added MIDebugSession::m_shuttingDown. There are no
related crash reports and this data member seems unnecessary. When
MIDebugSession does not finish successfully, it is destroyed via
MIDebugSession::killDebuggerImpl(), which sets s_dbgNotStarted state and
disables the stopDebugger() call in ~MIDebugSession().

I think the leak should be fixed in the test instead. If the test does
not use DebugController, it should provide a minimal replacement. If a
future proper leak fix triggers a crash again, consider replacing the
`if` block that contains the `stopDebugger()` call with
`killDebuggerNow()` in ~MIDebugSession() instead of reintroducing
MIDebugSession::m_shuttingDown.

This reverts commit 1256df11.