lift main-thread limit on iOS bots

I noticed this today while working on something else.
Looks like we added --threads 0 hoping it'd help something
on iOS a long time ago, and we decided it had no effect,
but didn't revert the change.

Bug: skia:6748
Change-Id: I4fb6722ae79ca21d1f7ee96bcb31b8ffb5319143
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/202010
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Stephan Altmueller <stephana@google.com>
2 files changed