fix(db-mongodb): remove duplicative indexing of timestamps (#11028)

### What?
This PR removes a pair unnecessary calls to `schema.index` against the
timestamp fields. The issue is when a user sets `indexSortableFields` as
this is what will ultimately pass the predicate which then creates
duplicate indexes.

### Why?
These calls are redundant as `index` is [already
passed](https://github.com/payloadcms/payload/blob/main/packages/db-mongodb/src/models/buildSchema.ts#L69)
to the underlying fields base schema options in the process of
formatting and will already be indexed.

These warnings were surfaced after the bump to mongoose to version 8.9.5
as [in 8.9.3 mongoose began throwing these warnings to indicate
duplicative
indexes](https://github.com/Automattic/mongoose/releases/tag/8.9.3).

### How?
By removing these calls and, as a result, silencing the warnings thrown
by mongoose.

packages/db-mongodb/src/models/buildCollectionSchema.ts

@@ -34,11 +34,6 @@ export const buildCollectionSchema = (
     schema.index(indexDefinition, { unique: true })
   }
 
-  if (payload.config.indexSortableFields && collection.timestamps !== false) {
-    schema.index({ updatedAt: 1 })
-    schema.index({ createdAt: 1 })
-  }
-
   schema
     .plugin<any, PaginateOptions>(paginate, { useEstimatedCount: true })
     .plugin(getBuildQueryPlugin({ collectionSlug: collection.slug }))