CI Drop PR trigger for Ubuntu16.04 arm64 Debug Build leg

[sdmaclea]

Due to CI capacity, it would be better to not run these on every PR.

@weshaggard Please assign to me. I'll take care of it after dotnet/corefx#28250 merges.
@eerhardt I'll also take care of sharing more code as you suggested.

[sdmaclea]

The arm64 & x86 CI legs are currently only running build-native.sh on PRs and Merges.
The arm & armel legs are running ./build.sh and building the whole runtime managed + native.

arm64 is not able to do that until official packages are released.

In light of that, I will not try to simplify the groovy yet.