Upgrade/Install: Revert relocation of Hello Dolly plugin.
Reverts Hello Dolly changes moving the plugin to a containing folder. Removes the upgrade_690() function in it's entirety as the upgrade routine is no longer required.
Fully reverted commits: [60666], [60670], [60716], [60725]; partially reverts [60721].
Porps johnbillion, whyisjake, SirLouen, mindctrl, afragen, jorbin, Otto42, swissspidy, welcher, davidbaumwald.
See #53323.