As discussed in #22446 we need to remove the plg_fields_repeatable plugin as soon as #22446 is merged into 3.10-dev

Background: Pull request #22446 implements a new custom field type plg_fields_subfields which is capable of using predefined custom fields for use as sub fields of a subform containing those. This functionality is basically an expansion of the functionality of the plg_fields_repeatable type. Hence, to keep the Joomla! core clean, we need to remove the plg_fields_repeatable plugin.

What needs to be done is that the plg_fields_repeatable plugin needs to be removed for all new installations of Joomla!, and for all existing installations that do not use that plugin (so have no repeatable custom fields). For installations that use it, we need to either provide a transitioning-script, or we need to explain the user how to switch from the repeatable type to the subfields type. To accomplish this, we maybe first need to declare the plg_fields_subfields type as deprecated in a first step.

A first idea of how to remove the repeatable type from core and how to build a transitioning script can be found in this diff continga@71f831d
That diff was previously part of the beforementioned PR, but we decided to do the plg_fields_repeatable stuff in a separate issue and keep it outside of the scope of the plg_fields_subfields PR. That is the reason this issue got created.

So in a first place, we need to decide on the actions that need to be taken in regards to the plg_fields_repeatable plugin when #22446 got merged, and when we have decided and when #22446 is merged, we need to create an additional PR for this.