roles: remove all luci/uhttp packagages on low_mem / low_flash

[Noki]

This goal of this PR is to reduce the image size for model with low_mem and low_flash by removing luci packages and unused packages which are currently included in the images. However there are some issues

• the solution is not elegant and does not distinguish between APs and core routers
• the OWM script should be present and work on core routers, but should not be installed on APs
  • the OWM script is included in luci-app-falter-owm, but the package brings in luci stuff in addition
• core routers flashed with the current version of this PR have issues to bring up SSH in a reasonable time, almost as randomness is missing. The situation resolves after several minutes or sometimes hours.