Clean up logic that was deprecated in Hydra 1.0

[omry]

This is a master task for cleanup of things deprecated in Hydra 1.0.

• Remove ObjectConf and deprecated params support from hydra.utils.instantiate() 1.0 upgrade page
• Default pacakge group should be _group_ (doc).
• @hydra.main config_path cleanup (1.0 Upgrade page)
• Remove strict flag (1.0 Upgrade page)