SkDocument: remove unused fDoneProc

Change-Id: I9a0739992e90a0a6d44a75b0b570097553343f1d
Reviewed-on: https://skia-review.googlesource.com/92141
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
7 files changed