fix: memory leak in terminal editor

[SimonSiefke]

Fixes a memory leak in terminal editor by removing the focus listener when clearInput is called.

Before

When moving a terminal from panel to editor area 97 times, a function seems to be created each time at terminalEditor.ts:87:64 which is never removed.

{
  "namedFunctionCount3": [
    {
      "count": 99,
      "delta": 97,
      "name": "anonymous",
      "sourceLocation": "out/vs/workbench/workbench.desktop.main.js:2841:56564",
      "originalLocation": "src/vs/workbench/contrib/terminal/browser/terminalEditor.ts:87:64",
      "originalName": "TerminalEditor.setInput"
    }
  ],
  "isLeak": true
}

After

No more leak is detected.

{
  "namedFunctionCount3": [],
  "isLeak": false
}

[meganrogge]

Thank you!