fix(build): remove ungated >= Swift 5.4 features to allow building on Xcode 12 (#1737)

Author: atierian

AmplifyPlugins/DataStore/AWSDataStoreCategoryPlugin/Storage/SQLite/ModelSchema+SQLite.swift

@@ -118,10 +118,13 @@ extension ModelSchema {
     /// Create SQLite indexes corresponding to secondary indexes in the model schema
     func createIndexStatements() -> String {
         var statement = ""
-        for index in indexes {
-            if case let .index(fields, name) = index, let name = name {
-                statement += CreateIndexStatement(modelSchema: self, fields: fields, indexName: name).stringValue
-            }
+        for case let .index(fields, name?) in indexes {
+            statement += CreateIndexStatement(
+                modelSchema: self,
+                fields: fields,
+                indexName: name
+            )
+            .stringValue
         }
         return statement
     }