Remove ESC key fallback mapping

The escape key has a fallback "back" behaviour. On ChromeOS
or tablets with bluetooth/attached keyboards, pressing the ESC
key can unexpectedly close applications. This removes the
fallback mapping so that ESC does nothing.

Bug: 71907807
Test: Manual testing on Marlin
Change-Id: I747f0bd743ec117e6ae47fae527600a3ab5690ba