fix: disable open keymap split editor from toolbar

[kittaakos]

Motivation

The complete support for split editor is yet to be added to IDE2. For now, this commit turns off opening the keymaps in the JSON split editor. So that it cannot get the primary IDE2 layout into an inconsistent state.

Change description

Other information

Closes arduino/arduino-ide#1850

Reviewer checklist

• [ ] PR addresses a single concern.
• [ ] The PR has no duplicates (please search among the Pull Requests before creating one)
• [ ] PR title and description are properly filled.
• [ ] Docs have been added / updated (for bug fixes / features)

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