ssh remoting: Fix ssh process not being cleaned up when connection is closed (#18623)

We introduced a memory leak in #18572, which meant that `Drop` was never
called on `SshRemoteConnection`, meaning that the ssh process kept
running

Co-Authored-by: Thorsten <thorsten@zed.dev>

Release Notes:

- N/A

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Co-authored-by: Thorsten <thorsten@zed.dev>