ueventd: remove character device symlinks (/dev/usb/*)

While refactoring ueventd, I was looking for code the character device
symlinks (/dev/usb/*) that ueventd creates, such that I could test it
on a real device.  I found none in our tree, and history showing Xoom,
which was last supported years ago, was the last user.  If this code
is in fact obsolete, let's remove it.

Test: boot bullhead
Test: init unit tests

Change-Id: I601f7165eb06d36b31b6dcf69ee9e0a449d81a96