wlan: Remove hdd context validation in wlan_hdd_disconnect

Propagation from qcacld-3.0 to prima

If driver is removing while STA is in connected state, disconnect
is not happening because hdd context validation will not be success
while driver unload is in progress.

To address this issue, remove validation of hdd context in
wlan_hdd_disconnect to allow disconnection while driver unload
is in progress.

Change-Id: Ia10aa9237a30490f3085458ab38ce27c4eeebaf0
CRs-Fixed: 2284772