Remove MSK from key source as it is not supported in model.

Author: sasubrata

device/macsec/mka/basic.yaml

@@ -149,15 +149,13 @@ components:
       properties:
         choice:
           description: |-
-              Key source. Choose one from PSK or MSK.
+              Key source. Current choice is PSK only.
           type: string
           default: psk
           x-field-uid: 1
           x-enum:
             psk:
               x-field-uid: 1
-            msk:
-              x-field-uid: 2
         psks:
           description: |-
             PSK chain.