[NFC] TypeCheckDeclPrimary.cpp: remove unused `isGenericType`
This change fixes a warning encountered when compiling this file:
```
swift/lib/Sema/TypeCheckDeclPrimary.cpp:475:8: warning: unused variable 'isGenericType' [-Wunused-variable]
bool isGenericType = isa<NominalTypeDecl>(decl);
```
