Clean our src/etc of old files

Some of these have long since expired, some are no longer in use now that we've jettisoned the makefiles, but none of them should be needed any more.
rust-lang · bors · Feb 8, 2017 · Jan 1, 2017 · Jan 28, 2017 · Jan 28, 2017
commit ffd3070cc9ea6d845865f11d70c220da40720d7e
diff --git a/src/etc/Dockerfile b/src/etc/Dockerfile
diff --git a/src/etc/apple-darwin.supp b/src/etc/apple-darwin.supp
diff --git a/src/etc/check-sanitycheck.py b/src/etc/check-sanitycheck.py
diff --git a/src/etc/check-summary.py b/src/etc/check-summary.py
diff --git a/src/etc/get-stage0.py b/src/etc/get-stage0.py
diff --git a/src/etc/local_stage0.sh b/src/etc/local_stage0.sh