Disable the chrome_sdk stage for variants of grouped builders.
Disable the chrome_sdk stage for variants of grouped builders.

Right now we enable chrome_sdk stage for x86-alex_he, x86-zgb_he, and
parrot_ivb. This unnecessarily slows down builds, as developers can just
use the x86-alex / x86-zgb / parrot SDK binaries for this.

BUG=chromium:341219
TEST=Verify parrot-release-group builds are much faster.

Change-Id: I2a1ddac9d2269b3c9c20c0ddd3b3df16608f7dee
Reviewed-on: https://chromium-review.googlesource.com/185073
Reviewed-by: Matt Tennant <46e612415b446ccde19638e4035892001c237942@chromium.org>
Commit-Queue: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite