clean up more dead code

  - SkSHA1 is unused
  - SkRunnable is obsolete now that we have std::function

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1705583003

Review URL: https://codereview.chromium.org/1705583003
14 files changed