Removing unwanted variable (#23865)

Author: osaghaso

specification/eventgrid/data-plane/Microsoft.Communication/stable/2018-01-01/AzureCommunicationServices.json

@@ -901,10 +901,6 @@
       "description": "Custom Context of Incoming Call",
       "type": "object",
       "properties": {
-        "customHeaders": {
-          "type": "string",
-          "description": "Custom Headers for incoming call"
-        },
         "sipHeaders": {
           "type": "object",
           "description": "Sip Headers for incoming call",