MT#57811 adjust 15761 drop index to "if exists"

* this addresses an issue caused by 15739_not_replicated script
  that when not applied on the other replicated node, causes
  this script to break replication. "drop index if not exists"
  in this script addresses the issue and makes it so the both
  scenarios are covered.

Change-Id: I8713d38ce3c4eab76b1d0b28adc6c0bd4c8260e1

db_scripts/diff/15761.up

@@ -1,5 +1,5 @@
 USE kamailio;
 
 ALTER TABLE acc
-    DROP KEY method_callid_idx,
+    DROP KEY IF EXISTS method_callid_idx,
     ADD KEY method_idx (method);