fix: remove unused import

Author: long76

servlet/src/main/java/io/grpc/servlet/GrpcServlet.java

@@ -20,7 +20,6 @@
 import io.grpc.ExperimentalApi;
 import java.io.IOException;
 import java.util.List;
-import javax.servlet.ServletException;
 import javax.servlet.http.HttpServlet;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;