[NFC] Remove duplicate code in SBTypeCategory

TypeCategoryImpl has its own implementation of these, so it makes no
sense to have the same logic inlined in SBTypeCategory.

There are other methods in SBTypeCategory that are directly implemented
there, instead of delegating to TypeCategoryImpl (which IMO kinda
defeats the point of having an "opaque" member pointer in the SB type),
but they don't have equivalent implementations in TypeCategoryImpl, so
this patch only picks the low-hanging fruit for now.