Do not import generated class in model classes

It's not necessary and can lead to issues with other build runners that
can't find the yet to be generated import. So don't do it in the repo
to avoid it being interpreted as best practice.

Author: greenrobot-team

generator/integration-tests/basics/lib/lib.dart

@@ -1,7 +1,6 @@
 import 'dart:typed_data';
 
 import 'package:objectbox/objectbox.dart';
-import 'objectbox.g.dart';
 export 'other.dart';
 
 @Entity()

generator/integration-tests/final/lib/lib.dart

@@ -2,8 +2,6 @@ import 'dart:typed_data';
 
 import 'package:objectbox/objectbox.dart';
 
-import 'objectbox.g.dart';
-
 // Test all final fields; Id must be self-assigned.
 @Entity()
 class A {

generator/integration-tests/getters/lib/lib.dart

@@ -1,7 +1,5 @@
 import 'package:objectbox/objectbox.dart';
 
-import 'objectbox.g.dart';
-
 /// Test annotations on getters are applied to properties when using
 /// getter + setter combos to create synthetic fields.
 @Entity()

generator/integration-tests/indexes/lib/lib.dart

@@ -1,5 +1,4 @@
 import 'package:objectbox/objectbox.dart';
-import 'objectbox.g.dart';
 
 @Entity()
 class A {

generator/integration-tests/relations/lib/lib.dart

@@ -1,5 +1,4 @@
 import 'package:objectbox/objectbox.dart';
-import 'objectbox.g.dart';
 
 @Entity()
 class A {

objectbox/example/flutter/event_management_tutorial/event_manager/lib/model.dart

@@ -1,5 +1,4 @@
 import 'package:objectbox/objectbox.dart';
-import 'objectbox.g.dart';
 
 @Entity()
 class Task {

objectbox/example/flutter/event_management_tutorial/many_to_many/lib/model.dart

@@ -1,5 +1,4 @@
 import 'package:objectbox/objectbox.dart';
-import 'objectbox.g.dart';
 
 @Entity()
 class Task {

objectbox/example/flutter/objectbox_demo/lib/model.dart

@@ -1,8 +1,6 @@
 import 'package:intl/intl.dart';
 import 'package:objectbox/objectbox.dart';
 
-import 'objectbox.g.dart';
-
 // ignore_for_file: public_member_api_docs
 
 @Entity()

objectbox/example/flutter/objectbox_demo_relations/lib/model.dart

@@ -1,8 +1,6 @@
 import 'package:intl/intl.dart';
 import 'package:objectbox/objectbox.dart';
 
-import 'objectbox.g.dart';
-
 @Entity() // Signals ObjectBox to create a Box for this class.
 class Tag {
   // Every @Entity requires an int property named 'id'

objectbox/example/flutter/objectbox_demo_sync/lib/model.dart

@@ -1,8 +1,6 @@
 import 'package:intl/intl.dart';
 import 'package:objectbox/objectbox.dart';
 
-import 'objectbox.g.dart';
-
 // ignore_for_file: public_member_api_docs
 
 @Entity()