chore: resolve pmd

Author: ZorinAnton

isthmus/src/main/java/io/substrait/isthmus/SqlConverterBase.java

@@ -1,6 +1,5 @@
 package io.substrait.isthmus;
 
-import io.substrait.extension.DefaultExtensionCatalog;
 import io.substrait.extension.SimpleExtension;
 import org.apache.calcite.config.CalciteConnectionConfig;
 import org.apache.calcite.config.CalciteConnectionProperty;