fix (ledvance_smart_plabl100x25b): simplify light availability

Lights contain logic to fall back on standard dps when 51 is missing, but
making the availability false when dp 51 is missing defeats this.

Observation from past experience is that the light reports current state
using the standard dps, but control must be via dp51 to work.

Reviewed as part of issue #4024

custom_components/tuya_local/devices/ledvance_smart_plabl100x25b.yaml

@@ -21,18 +21,6 @@ entities:
       - id: 20
         type: boolean
         name: available
-        mapping:
-          - dps_val: true
-            value: true
-            constraint: raw_switch
-            conditions:
-              - dps_val: null
-                value: false
-      - id: 51
-        type: base64
-        optional: true
-        name: raw_switch
-        hidden: true
       - id: 51
         type: base64
         mask: "000100000000000000000000"
@@ -97,18 +85,6 @@ entities:
       - id: 20
         type: boolean
         name: available
-        mapping:
-          - dps_val: true
-            value: true
-            constraint: raw_switch
-            conditions:
-              - dps_val: null
-                value: false
-      - id: 51
-        type: base64
-        optional: true
-        name: raw_switch
-        hidden: true
       - id: 51
         type: base64
         mask: "000200000000000000000000"