MDL-40401 Libraries: load_temp_role() and remove_temp_roles() can not be used any more.
MDL-40402 libraries: Replace get_system_context() with context_system::instance() in core
MDL-40402 unittests: Make sure get_system_context() throws a deprecated warning
MDL-40402 libraries: Deprecate get_system_context()
MDL-40402 libraries: update the mistake in comment
MDL-40403 libraries: Deprecate get_parent_contexts()
MDL-40403 tests: Fix pointless assertion