Some GN-related recipe cleanup.

All our bots but the iOS ones are on GN now.  This cleans up a bunch of GYP and other obsolete stuff.

  - Nothing's using default_flavor.py any more except as a base class.
  - There are no -CMake, -Shared or -VisualBench bots anymore.
  - Only the iOS bots care about GYP_DEFINES.

You'll see the PDFium bot's GYP_DEFINES change, but that doesn't matter... it's using PDFium's own (GN) build system, and it ignores GYP_DEFINES.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3567

Change-Id: I699e10f013ea77df4dcaa1cb559c51c5bf55dfdb
Reviewed-on: https://skia-review.googlesource.com/3567
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
20 files changed