Remove unneeded SkPackedUnicharID

Change-Id: I85334081f3b114dc0855adcbee4156247fd457bb
Reviewed-on: https://skia-review.googlesource.com/c/191720
Auto-Submit: Herb Derby <herb@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>