Restarting sessions was broken in #33273 when we moved away from calling
`kill` in the shutdown sequence. This PR re-adds that `kill` call so
that old debug adapter processes will be cleaned up when sessions are
restarted within Zed. This doesn't re-introduce the issue that motivated
the original changes to the shutdown sequence, because we still send
Disconnect/Terminate to debug adapters when quitting Zed without killing
the process directly.

We also now remove manually-restarted sessions eagerly from the session
list.

Closes #33916 

Release Notes:

- debugger: Fixed not being able to restart sessions for Debugpy and
other adapters that communicate over TCP.
- debugger: Fixed debug adapter processes not being cleaned up.

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Co-authored-by: Remco Smits <djsmits12@gmail.com>