Remove change to isGeneric on 5.10

The semantics we need here are fine enough with the previous code; The `isGeneric` method does not exist on 5.10 so let's keep using what worked well before already.

Author: ktoso

lib/IRGen/GenDistributed.cpp

@@ -693,7 +693,7 @@ void DistributedAccessor::emit() {
   }
 
   // Add all of the substitutions to the explosion
-  if (Target->isGeneric()) {
+  if (auto *genericEnvironment = Target->getGenericEnvironment()) {
     // swift.type **
     llvm::Value *substitutionBuffer =
         IGF.Builder.CreateBitCast(substitutions, IGM.TypeMetadataPtrPtrTy);