Draft: Remove forced asset compilation via webpack support

This is just a basis for discussion: how to move us away from webpack and asset-packager implementation details and rely on simpler integration points with whatever gitlab-rails chooses today as its asset packer.

I made two changes:

• remove the make webpack-compile entry point since it is more stable to just use make assets-compile instead, which relies on the better maintained Rake task in gitlab-rails
• remove the code paths in webpack.sh that ran custom logic to perform a full asset compilation; this code would have to been maintained in lockstep with any changes to asset packaging in gitlab-rails, which makes for a brittle integration point