Revert "Use read/write engine lock to guard operations against resets (elastic#124635)" (elastic#125915)

This reverts commit 4aa7ce5.

Author: tlrx

server/src/main/java/org/elasticsearch/index/engine/Engine.java

@@ -2371,7 +2371,7 @@ public record FlushResult(boolean flushPerformed, long generation) {
     }
 
     /**
-     * Ensures the engine is in a state that it can be closed by a call to {@link IndexShard#resetEngine(Consumer<Engine>)}.
+     * Ensures the engine is in a state that it can be closed by a call to {@link IndexShard#resetEngine()}.
      *
      * In general, resetting the engine should be done with care, to consider any
      * in-progress operations and listeners (e.g., primary term and generation listeners).