Hide legacy widgets when not already in use

[JakePT]

Is your enhancement related to a problem? Please describe.
When using the block-based widgets editor both legacy widgets and blocks are available for Related Posts and Facets. This could be confusing as there are two options for the same thing.

Describe the solution you'd like
Following the pattern of WordPress core's legacy widgets that became blocks, we should not register the legacy widgets on sites that are not already using the legacy widget somewhere and are not using the classic widgets plugin, or similar. This would keep the widgets around for those who need them, while only making blocks available for all new sites.