remove references to `Blockly.FieldColour` from keyboard-navigation

[maribethb]

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Component

keyboard-navigation

Description

The keyboard navigation attempts to patch Blockly.FieldColour. That field no longer exists, so even referencing it causes webpack warnings.

To fix

1. In navigation_controller.ts, remove the lines that attempt to add or remove the onShortcut handler to Blockly.FieldColour.
2. In the same file, remove the now-unused fieldColourHandler function.

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Reproduction steps

1. Run npm run build in plugins/keyboard-navigation
2. Notice warnings about how Blockly.FieldColour doesn't exist

Stack trace

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Screenshots

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