Revert "Issue #1906810 by dawehner, damiankloip, tstoeckler, kgoel, fubhy, jrglasgow, xjm, Gaelan, socketwench: Require type hints for automatic entity upcasting."

This reverts commit c9e73f7def15f7a03f462b2b485cf1f6fa794a9e.
Issue #1906810 by dawehner, damiankloip, tstoeckler, kgoel, fubhy, jrglasgow, xjm, Gaelan, socketwench: Require type hints for automatic entity upcasting.
Issue #2143291 by plach, alexpott, xjm, effulgentsia, pwolanin, swentel | yched: Clarify handling of field translatability.
Issue #2124957 by Wim Leers, catch: Replace 'content' cache tag with 'rendered' and use it sparingly.
Issue #2261083 by amitgoyal, joshi.rohit100, er.pushpinderrana, aschmoe: Move mention of System Help block from Help module help to System module help
Issue #2287319 by martin107: Document exceptions thrown in KernelTestBase
Issue #2191445 by jhodgdon: Fixed Database abstraction layer topic / landing page needs more info.
Issue #2234439 by jhodgdon, cosmicdreams, xjm: Add a section for the plugin system to the API doc landing page.