Remove books.es from version control and add to .gitignore

Author: mridula-s109

.gitignore

@@ -73,3 +73,4 @@ x-pack/plugin/esql/src/main/generated-src/generated/
 # JEnv
 .java-version
 x-pack/plugin/search-business-rules/src/main/java/org/elasticsearch/xpack/searchbusinessrules/retriever/books.es
+x-pack/plugin/search-business-rules/src/main/java/org/elasticsearch/xpack/searchbusinessrules/retriever/books.es