Revert "Introduce system index APIs for Kibana (#52385)" (#53912)

This reverts commit 4c0e8f1.

It should be re-added once #53909 is addressed.

Author: rjernst

build.gradle

@@ -222,8 +222,8 @@ task verifyVersions {
  * after the backport of the backcompat code is complete.
  */
 
-boolean bwc_tests_enabled = true
-final String bwc_tests_disabled_issue = "" /* place a PR link here when committing bwc changes */
+boolean bwc_tests_enabled = false
+final String bwc_tests_disabled_issue = "https://github.com/elastic/elasticsearch/pull/53912" /* place a PR link here when committing bwc changes */
 if (bwc_tests_enabled == false) {
   if (bwc_tests_disabled_issue.isEmpty()) {
     throw new GradleException("bwc_tests_disabled_issue must be set when bwc_tests_enabled == false")