Remove MapperService.getObjectMapper() (#68403)

This is still used in a couple of tests, and can be replaced by
mapperService.mappingLookup().objectMappers().get()

server/src/main/java/org/elasticsearch/index/mapper/MapperService.java

@@ -414,10 +414,6 @@ public class MapperService extends AbstractIndexComponent implements Closeable {
             this.mapper.mappers().fieldTypes().filter(MappedFieldType::eagerGlobalOrdinals);
     }
 
-    public ObjectMapper getObjectMapper(String name) {
-        return this.mapper == null ? null : this.mapper.mappers().objectMappers().get(name);
-    }
-
     /**
      * Return the index-time analyzer associated with a particular field
      * @param field                     the field name

server/src/test/java/org/elasticsearch/index/search/NestedHelperTests.java

@@ -93,7 +93,7 @@ public class NestedHelperTests extends ESSingleNodeTestCase {
     }
 
     private static NestedHelper buildNestedHelper(MapperService mapperService) {
-        return new NestedHelper(mapperService::getObjectMapper, field -> mapperService.fieldType(field) != null);
+        return new NestedHelper(mapperService.mappingLookup().objectMappers()::get, field -> mapperService.fieldType(field) != null);
     }
 
     public void testMatchAll() {

test/framework/src/main/java/org/elasticsearch/index/mapper/MapperServiceTestCase.java

@@ -493,7 +493,7 @@ public abstract class MapperServiceTestCase extends ESTestCase {
         when(searchExecutionContext.getIndexAnalyzers()).thenReturn(mapperService.getIndexAnalyzers());
         when(searchExecutionContext.getIndexSettings()).thenReturn(mapperService.getIndexSettings());
         when(searchExecutionContext.getObjectMapper(anyString())).thenAnswer(
-            inv -> mapperService.getObjectMapper(inv.getArguments()[0].toString()));
+            inv -> mapperService.mappingLookup().objectMappers().get(inv.getArguments()[0].toString()));
         when(searchExecutionContext.simpleMatchToIndexNames(anyObject())).thenAnswer(
             inv -> mapperService.simpleMatchToFullName(inv.getArguments()[0].toString())
         );