chore(engine): drop handling JPA entities as variables (camunda#3468)

* from engine, engine-spring, spring-boot-starter
* adds a migration test 

Related to camunda#3447

engine-spring/core/pom.xml

@@ -64,11 +64,6 @@
       <!--  do not expose -->
       <scope>provided</scope>
     </dependency>
-    <dependency>
-      <groupId>org.hibernate.javax.persistence</groupId>
-      <artifactId>hibernate-jpa-2.0-api</artifactId>
-      <scope>provided</scope>
-    </dependency>
 
     <!-- servlet API -->
     <dependency>
@@ -171,10 +166,6 @@
         <artifactId>maven-surefire-plugin</artifactId>
         <configuration>
           <redirectTestOutputToFile>true</redirectTestOutputToFile>
-          <classpathDependencyExcludes>
-            <!-- JPA 2.0 is for compile time, upon testing we use the environment defined by the hibernate and spring context (e.g. JPA 2.2) -->
-            <classpathDependencyExclude>org.hibernate.javax.persistence:hibernate-jpa-2.0-api</classpathDependencyExclude>
-          </classpathDependencyExcludes>
           <dependenciesToScan>
             <dependency>org.camunda.bpm:camunda-bpm-archunit</dependency>
           </dependenciesToScan>

engine-spring/core/src/main/java/org/camunda/bpm/engine/spring/SpringTransactionsProcessEngineConfiguration.java

@@ -35,7 +35,6 @@
 import org.camunda.bpm.engine.impl.interceptor.CommandInterceptor;
 import org.camunda.bpm.engine.impl.interceptor.LogInterceptor;
 import org.camunda.bpm.engine.impl.interceptor.ProcessApplicationContextInterceptor;
-import org.camunda.bpm.engine.impl.variable.serializer.jpa.EntityManagerSession;
 import org.camunda.bpm.engine.repository.DeploymentBuilder;
 import org.springframework.core.io.ByteArrayResource;
 import org.springframework.core.io.ContextResource;
@@ -118,15 +117,6 @@ protected void initTransactionContextFactory() {
     }
   }
 
-  @Override
-  protected void initJpa() {
-    super.initJpa();
-    if (jpaEntityManagerFactory != null) {
-      sessionFactories.put(EntityManagerSession.class,
-              new SpringEntityManagerSessionFactory(jpaEntityManagerFactory, jpaHandleTransaction, jpaCloseEntityManager));
-    }
-  }
-
   protected void autoDeployResources(ProcessEngine processEngine) {
     if (deploymentResources!=null && deploymentResources.length>0) {
       RepositoryService repositoryService = processEngine.getRepositoryService();