Remove flag_couldbechild

Remove flag_couldbechild

Now that a provider is no longer put into the store until after it has
been activated we don't need flag_couldbechild any more. This flag was
used to indicate whether a provider was eligible for conversion into a
child provider or not. This was only really interesting for predefined
providers that were automatically created.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15854)