Fix write transaction shard stats

With the changes in ask protocol due to to tell based
integration, shards stats had incorrectly reported
write transaction counts. Since backend doesnt care
about the difference between write-only and read-write
transactions and all are read-write from backend point of view
we can safely remove writeOnly counter.

Change-Id: I18046ad3b229755bbd0daa788a3fe62f59e4140d
Signed-off-by: Tomas Cere <tomas.cere@pantheon.tech>