Remove HID usage mapping for stylus buttons in Generic.kl

... until we figure out a way to determine if a device supports a
specific HID usage code. Right now, having a usage code mapping means we
automatically assume the device can report the key code. Assuming a
device can support a stylus keycode makes the device get classified as a
stylus, leading to widespread misclassifications.

Bug: 291529805
Test: atest inputflinger_tests
Change-Id: I580514bdcbbfbe93df83fa3faa6262077e881243