Remove entry for version 18.0.0b40

Removed version 18.0.0b40 entry from HISTORY.rst.

Author: m-chau

src/aks-preview/HISTORY.rst

@@ -12,9 +12,6 @@ To release a new version, please select a new version number (usually plus 1 to
 Pending
 +++++++
 * Add option `Windows2025` to `--os-sku` for `az aks nodepool add`.
-
-18.0.0b40
-+++++++
 * Update user prompt for `az aks upgrade` command to clarify the implication of the `--control-plane-only` options.
 
 18.0.0b39
@@ -2002,4 +1999,4 @@ Pending
 +++++
 
 * new feature `enable-cluster-autoscaler`
-* default agentType is VMSS
+* default agentType is VMSS