Remove shortcut for opening Copilot suggestions pane

[jessehouwing]

Removed the 'Open Copilot (additional suggestions in separate pane)' shortcut for macOS, Windows, and Linux.

This feature has been removed from all copilot extensions some time ago.

Why:

Closes:

What's being changed (if available, include any code snippets, screenshots, or gifs):

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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Source	Review	Production	What Changed
copilot/reference/keyboard-shortcuts.md	fpt ghec	fpt ghec

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[Copilot]

Pull request overview

Removes outdated keyboard shortcuts for opening the separate Copilot suggestions pane across supported IDE shortcut tables, reflecting that the feature was removed from Copilot extensions.

Changes:

• Removed the “Open GitHub Copilot (additional suggestions in separate pane)” shortcut rows for JetBrains IDEs (macOS/Windows/Linux).
• Removed the corresponding VS Code shortcut rows and the github.copilot.generate command entry (macOS/Windows/Linux).