Remove macro in the threaded backend

This is a smaller CL distilled from our init-once CL (87784).
Note that the thread alloc is unused here but it will be used by
init-once.

Bug: skia:
Change-Id: If4bc2febfb3fbcd85bf973a01e60b9b64c873a19
Reviewed-on: https://skia-review.googlesource.com/92621
Commit-Queue: Yuqian Li <liyuqian@google.com>
Reviewed-by: Herb Derby <herb@google.com>
2 files changed