Disable the address sanitizer in the linux_unopt build (#165661)

This is causing issues on LUCI workers that were upgraded to a new
version of Ubuntu.

See https://github.com/flutter/flutter/issues/165594
See https://github.com/flutter/flutter/pull/165620

engine/src/flutter/ci/builders/linux_unopt.json

@@ -16,7 +16,6 @@
                 "debug",
                 "--unoptimized",
                 "--prebuilt-dart-sdk",
-                "--asan",
                 "--dart-debug",
                 "--rbe",
                 "--no-goma"