[OpenACC] Remove unreachable code

As stated on the commit by @shafik, the previous patch left in some code
from development.  This removes it, as it is unreachable.

Author: erichkeane

clang/lib/Sema/SemaOpenACCClause.cpp

@@ -1360,7 +1360,6 @@ ExprResult CheckGangKernelsExpr(SemaOpenACC &S,
   }
   case OpenACCGangKind::Static:
     return CheckGangStaticExpr(S, E);
-    return ExprError();
   }
   llvm_unreachable("Unknown gang kind in gang kernels check");
 }