KeyboardInputMapper: Remove unused "stem" key mapping

There is code in KeyboardInputMapper to statically remap "stem" keys
based on one device's IDC file.

I'm not sure why this was added, but there are no usages of these key
remappings that I could find, so I'm removing these to simplify the
code.

Bug: 245989146
Test: None
Change-Id: I8745de89c3631d7dc0f2c262ff5eb97c55f40ec1