allow purge expired STS while loading credentials (#19905)

the reason for this is to avoid STS mappings to be
purged without a successful load of other policies,
and all the credentials only loaded successfully
are properly handled.

This also avoids unnecessary cache store which was
implemented earlier for optimization.

cmd/iam-store.go

@@ -532,16 +532,6 @@ func setDefaultCannedPolicies(policies map[string]PolicyDoc) {
 	}
 }
 
-// PurgeExpiredSTS - purges expired STS credentials.
-func (store *IAMStoreSys) PurgeExpiredSTS(ctx context.Context) error {
-	iamOS, ok := store.IAMStorageAPI.(*IAMObjectStore)
-	if !ok {
-		// No purging is done for non-object storage.
-		return nil
-	}
-	return iamOS.PurgeExpiredSTS(ctx)
-}
-
 // LoadIAMCache reads all IAM items and populates a new iamCache object and
 // replaces the in-memory cache object.
 func (store *IAMStoreSys) LoadIAMCache(ctx context.Context, firstTime bool) error {