Remove Enum.h error-only header

This is supposed to map to libbinder 'Enums.h', but this file has a
different name, and including it is an error. Also, it's a nice error
for this one case, but we don't get errors for other cases, and we
haven't seen any more reports of b/169874616 even though this is named
differently.

Bug: n/A
Test: N/A
Change-Id: I7182e54ed77fb247e81315adb82ad58c86014f32