Remove placeholders for shortcuts from keyboard experiment menu item labels

[gonfunko]

After RaspberryPiFoundation/blockly-keyboard-experimentation#594 is merged, the messages for keyboard-experiment-related menu items in core that include placeholders for the keyboard shortcut should be updated to remove those placeholders.

[gonfunko]

The corresponding hack to remove these in keyboard-experiment should in turn be removed after this is done.