Remove workarounds in build scripts for netcoreapp and netfx versionless (#1841)

* Remove workarounds in build scripts for netcoreapp and netfx versionless

* Don't pass down netcoreapp in yml builds

* PR Feedback and condition passing -framework arg when not empty in yml files

* Condition TargetGroup global property in helix.yml

* PR Feedback to not use targetgroup for allconfigurations in sendtohelix

* Change from PackageTesting to BuildAllConfigurations

diff --cc docs/coding-guidelines/project-guidelines.md
Simple merge

diff --cc eng/pipelines/coreclr/templates/crossgen-comparison-job.yml
Simple merge

diff --cc eng/pipelines/libraries/build-job.yml
Simple merge

diff --cc eng/pipelines/runtime.yml
Simple merge