Delete GenericKind_ in favor of GenericKind_DC

When deriving a Generic1 instance, we need to know what the last type variable of a data type is. Previously, there were two mechanisms to determine this information:

• GenericKind_, where Gen1_ stored the last type variable of a data type constructor (i.e., the tyConTyVars).
• GenericKind_DC, where Gen1_DC stored the last universally quantified type variable in a data constructor (i.e., the dataConUnivTyVars).

These had different use cases, as GenericKind_ was used for generating Rep(1) instances, while GenericKind_DC was used for generating from(1) and to(1) implementations. This was already a bit confusing, but things went from confusing to outright wrong after !6976 (closed). This is because after !6976 (closed), the deriving machinery stopped using tyConTyVars in favor of dataConUnivTyVars. Well, everywhere with the sole exception of GenericKind_, which still continued to use tyConTyVars. This lead to disaster when deriving a Generic1 instance for a GADT family instance, as the tyConTyVars do not match the dataConUnivTyVars. (See #21185 (closed).)

The fix is to stop using GenericKind_ and replace it with GenericKind_DC. For the most part, this proves relatively straightforward. Some highlights:

• The forgetArgVar function was deleted entirely, as it no longer proved necessary after GenericKind_'s demise.
• The substitution that maps from the last type variable to Any (see Note [Generating a correctly typed Rep instance]) had to be moved from tc_mkRepTy to tc_mkRepFamInsts, as tc_mkRepTy no longer has access to the last type variable.

Fixes #21185 (closed).